In light of last night's, erm, antics, I was surprised at the amount of people who don't know how to revert edits on pages. So I figured I'd make a tutorial to show you how it's done.
Look for where it says who last edited the page (it should be right underneath the "Edit" button), and hover over it with your mouse. Then, a longer list will pop up, and you'll see at the bottom a link that says "view full history". Click on that, and it will take you to the page's history. Find the edit you want to undo and click "undo".