As you save changes to your reports, the previous versions are kept around in case you need to restore back to a previous version. You can access the list of previous report versions by clicking on Configuration -> Version History.

As you save new versions, they'll be shown in this list. You can click on a previous version to pull up that previous version in the report editor. If you want to restore those previous changes, you can click on the Revert button (found where the Save button normally is). You can also do a Save As on a previous version to save a copy of the report instead of overwriting the report.