Well - if its worth fixing, you doas the editOP, and do a custom flag letting them know what to do.
That said - if you are going to do an extensive amount of work to fix an answer - you clearly think its worth it, and do it first. Alternatively, it ought to be fine to claim credit on a full answer with personal experiences and referencesthe necessary changes, rather than trying to polish up an answer where another user didn't make the effortsure its good enough and you did flag for undeletion.