I approved this suggested edit by mistake. I wanted to click on Reject tab but mistakenly I ended up clicking on Approve.
Is it possible to undo this action? Or what to do if something happen like this by mistake?
I've rejected the edit.
As it takes two people to approve (or reject) an edit then the danger of an accidental misclick is somewhat reduced.
The only things you can really do to prevent the edit going through are post in here or in chat to get others to review and reject the edit.
The fallback position is to monitor the question and if the edit does get accepted roll it back.