Is it good practice to edit answer to ones own question?
Suppose some one gives a good answer to an OP's question and the OP edits that answer. Is it considered a good practice?
Editing is always good practice so long as the edits are valid. There's really no special case that doesn't apply. In any event, with any edit on the site the revision is tracked and edits can be rolled back at any time.
So, please go ahead and edit.
Yes, you can edit answers to your own questions. You should keep in mind the same considerations that apply to editing in general:
The edit should be reasonably scoped to the answer. If you're writing a totally new answer (and you're not the author of the existing answer), post a new answer instead. (Remember, you can answer your own questions.)
If you have less than 2000 reputation, the edit should be significant, as someone (perhaps the answer's author, perhaps someone else) will have to review the edit and decide whether or not to accept it. The work of reviewing is justified by most edits (even ones that just add a few commas, improve capitalization throughout the post, or fix a couple misspellings), but if you would consider the edit to be small and insignificant, you shouldn't suggest it.
The edit should respect the original author of the post. You can say things in first person, especially if you're rewording something they've already said. But obviously, you should avoid putting words in their mouth that they wouldn't want to be there. For example, don't say, "I was a fool to write that last paragraph, as that only applies if..."
If the post is expressing a position on something, the edit should not change it to a different position. And unless the post is a rant or contains sections that are clearly and uncontroversially off-topic, you should not remove content that expresses a position either (though rewording it is often good, if it's ungrammatical or unclear).
An example of a position you probably shouldn't remove is: "Utility X is probably the best way for you to reconfigure your window manager."
An example of a position you almost definitely should remove is: "This is unrelated, but I've gotta say, team X is the best hockey team in all of Eastern Europe."