How do I revert to a previous version of a file using Lion's file versioning tool?
Open the file in question.
If the application you are using supports Versions (not all of them do yet) you will find a downward pointing triangle to the right of the title of the document at the top of the window.
Click on that and it will take you into Versions.  If there are previous saved version, you will be able to "scroll" back through them and select the one you want.


Some applications have not yet been updated to take advantage of Versions...  In that case, you will find that you are out of luck for now.

Bill