Click on Revisions under the article.
If you know which version you want, click on revert next to it to revert to that version.
If you are not sure which version you want, you can select two versions to compare and click on Show diffs. It will show what changed between the two versions, as will as show the newer version below. If you want to revert to the newer of the two, click on Revert to this revision at the bottom of the page.