As Mark Kirby mentioned in a comment there is no way to change a review after you've done it. A Recommend deletion vote can't be retracted, and neither can a delete vote cast by a user with delete vote privilege (I wish they could).

In a situation where you change your mind about whether a post should be deleted, there are a few options:

• A positive score on an answer in the low quality posts review queue will prevent deletion; even if the requisite six (I think) Recommend deletion votes are cast, a mod has to review the flag in the end. Users with 20k answer delete vote privilege can only vote to delete negatively scored posts, or posts with zero or less score through the queue. Only mods have unconditional delete vote privilege. Of course you should only upvote the post if you would have done so anyway.
• A negative score helps to delete the post by making it easier for delete votes to be cast. I always downvote posts I think should be deleted unless I'm encountering them in the low quality posts queue (which doesn't have a downvote button) and I am reasonably confident that they will be deleted without my downvoting them.

• Come to the Downboat (a chat room for moderation tasks) or Ask Ubuntu General Room to discuss the problem and solicit (un)delete votes. Users with 20k have the ability to vote to (un)delete answers, unless a mod has deleted them. Mods also hang out in chat, and they are omnipotent, obviously.
• Flag the post for moderator attention and ask for it to be undeleted
• Post on meta to advocate for the post (should rarely be necessary, but can be a last resort if you are unable to find any helpful folks in chat etc).

As Mark Kirby mentioned in a comment there is no way to change a review after you've done it. A Recommend deletion vote can't be retracted, and neither can a delete vote* cast by a user with delete vote privilege (I wish they could).

In a situation where a non-answer becomes an answer or for any other reason you change your mind about whether a post should be deleted, there are a few options:

• A positive score on an answer in the low quality posts review queue will prevent deletion; even if the requisite six (I think) Recommend deletion votes are cast, a mod has to review the flag in the end. Users with 20k answer delete vote privilege can only vote to delete* negatively scored posts, or posts with zero or less score through the queue. Only mods have unconditional delete vote privilege. Of course you should only upvote the post if you would have done so anyway.
• A negative score helps to delete the post by making it easier for delete votes* to be cast. I always downvote posts I think should be deleted unless I'm encountering them in the low quality posts queue (which doesn't have a downvote button) and I am reasonably confident that they will be deleted without my downvoting them.

• Come to the Downboat (a chat room for moderation tasks) or Ask Ubuntu General Room to discuss the problem and solicit (un)delete votes. Users with 20k have the ability to vote to (un)delete* answers, unless a mod has deleted them. Mods also hang out in chat, and they are omnipotent, obviously.
• Flag the post for moderator attention and ask for it to be undeleted
• Post on meta to advocate for the post (should rarely be necessary, but can be a last resort if you are unable to find any helpful folks in chat etc).

_{* three non-mod delete votes, which can only be cast on negatively scored answers (or zero scored answers through the queue), delete an answer, and three non-mod undelete votes, which can only be used if a mod did not delete the post, undelete it. For questions, the situation is a little different, but that seems out of the scope of this question. The main point of this note is to point out that it takes three users with => 20k to (un)delete an answer and even then it's a conditional privilege.}

The review in question was completed and the post survived

• Vote on the post.

• A positive score on an answer in the low quality posts review queue will prevent deletion; even if the requisite six (I think) Recommend deletion votes are cast, a mod has to review the flag in the end. Users with 20k answer delete vote privilege can only vote to delete negatively scored posts, or posts with zero or less score through the queue. Only mods have unconditional delete vote privilege. Of course you should only upvote the post if you would have done so anyway.

• A negative score helps to delete the post by making it easier for delete votes to be cast. I always downvote posts I think should be deleted unless I'm encountering them in the low quality posts queue (which doesn't have a downvote button) and I am reasonably confident that they will be deleted without my downvoting them.

• Consider editing the post to improve it, so that others are less likely to vote to delete it. I am saying this with caution however, as I am unsure in what circumstances this may have the drastic effect of completing the review without any further scrutiny of the post. I had thought this only happened when editing was selected as an option in the queue, but it seems like any edit can have that effect. Therefore, I recommend doing this only if you are very sure the post should not be deleted.

• Watch to see the outcome of the review. This is annoying, because it's not very easy to keep track of posts. You could favourite the question, but you might forget to check back or forget why you favourited it (as I do). I usually just press the back button to the review and leave the browser tab open until the review completes, which usually takes less than 24 hours.

The review in question was completed and the post survived.

Vote on the post

• A positive score on an answer in the low quality posts review queue will prevent deletion; even if the requisite six (I think) Recommend deletion votes are cast, a mod has to review the flag in the end. Users with 20k answer delete vote privilege can only vote to delete negatively scored posts, or posts with zero or less score through the queue. Only mods have unconditional delete vote privilege. Of course you should only upvote the post if you would have done so anyway.
• A negative score helps to delete the post by making it easier for delete votes to be cast. I always downvote posts I think should be deleted unless I'm encountering them in the low quality posts queue (which doesn't have a downvote button) and I am reasonably confident that they will be deleted without my downvoting them.

Consider editing the post

If you improve the post, perhaps rephrasing a "me too" statement or question at the beginning, others may be less likely to vote to delete it. I am saying this with caution however, as I am unsure in what circumstances this may have the drastic effect of completing the review without any further scrutiny of the post. I had thought this only happened when editing was selected as an option in the queue, but it seems like any edit can have that effect. Therefore, I recommend doing this only if you are very sure the post should not be deleted.

Watch to see the outcome of the review

This is annoying, because it's not very easy to keep track of posts. You could favourite the question, but you might forget to check back or forget why you favourited it (as I do). I usually just press the back button to the review and leave the browser tab open until the review completes, which usually takes less than 24 hours. If you have less than 10k reputation, you presumably won't be able to see the post if it has been deleted, but you'll probably see something like this which vaguely suggests that it has.