I accidentally clicked reject on the suggested edit on this question(https://stackoverflow.com/questions/5726269/). I can't find the button to change it to Approve?
There is no undo at the moment, I always planned to have undo gmail style. But we have resisted complexity here. Once you have undo, you have to add a slew of checks prior to undoing.
On SO the community will pick up and approve if you just made a mistake. On other sites, you can go and manually apply the edit if this was an honest mistake.
In general, you can't undo your review actions. The exceptions are when you can revert the changes your action effected - so, in the case of approving a bad suggested edit, you could rollback the edit. The other option would be to flag for moderator attention and ask them to override the edit.
These two actions may sound the same, but they aren't. Rolling back such an edit would still leave the person who suggested it with the two rep they earned for having it approved - in the case of spam or destructive edits, this would be bad. On the other hand, moderators have the ability to reject an edit after the fact, thus removing the reputation earned - with the caveat that there can't have been an intervening edit (or rollback). Given that, you'll have to pick one of the two options; you shouldn't both rollback and flag as the mod won't be able to do anything at that point.
Fortunately, in most cases, reviews require more than one person to act, so hopefully, if you approve a bad edit, someone else will catch it and it won't get through.