Anyone can revert a page to fix vandalism. All revisions of a page back to the first one are stored in the page history. To revert to an earlier version, just select and copy the text from the history, open the article for editing, paste it back in, and save it. When not dealing with obvious vandalism, reverting often is a bad strategy. It alienates other users and provokes edit wars. Stay cool, talk to the user in question directly, or try to resolve issues on the article's Talk page.
Please do not revert the same page more than three times within 24-hours (the three-revert rule). Doing so can lead to a temporary ban against you. Administrators and Rollbackers have a handy rollback feature that allows them to instant-revert vandalism by going to a user's contributions page. To revert only the most recent edit there is an undo link on the article history page or on the article diff page.