How can I do the cleanup badge task? I can't find any information about how to do it anywhere, I would be grateful if you can help me.
As the badge description, and this Main Meta post state - Complete your first Rollback.
A rollback is the reversion of an edit.
Click on the edit history of a post - where it says edited x hours ago.
This gives all the edits performed on the post
Click on the word Rollback on the state of the post you wish to restore.
If you have edit rights, the rollback occurs immediately. Otherwise it goes into the Suggested Edits review queue like any other proposed edit and your rollback only takes place if approved.