If I edit someone else's question and my edit goes into peer review, but then I realize that the edit was unnecessary, how do I undo the previous edit?
If your edit is accepted by the required users (two, on EL&U), you can see the text used by the post before your edit (click on the "edit [date]" link") and submit another suggested edit.
If your suggested edit is still not approved, then you need to wait it get approved or declined. If it gets declined, the post will probably appear as it was originally written by the user who wrote it.
Click "edit" on the OP's question. Look for the previous version and click on "roll back". (I'm not sure on this one) But it will still be peer reviewed before being approved.
Read the top answer here: What to do about incorrect edits?
At the top far right hand side where your username is, if you click on help you'll have a wealth of information available. I'm posting this link for any newbies who might be asking themselves the very same question.