Every time you edit and save your Article a new version is saved. You can view these versions and restore previous versions by following these steps:
Open the Article
From the Article Options menu (right-most green button on the top of the page), select "Changes"
From here you can preview any version of the document. You may also select a version by clicking the radio button on the right side of the version, and from there either restore that version to be the current version, or merge the older version into the current version, using the two buttons on the bottom of the page.