You can find a lot of information about SVN, the versioning tool that Mendix uses, online, you just need to google it.
Not sure, if roll back is possible out of box. But you can try the below steps.
1. create a branch from previous version of Mainline.
2. create a new branch from current version of Mainline (just in case if you dont want to loose your changes)
3. Once the branches are created, you can merge the branch created from step 1 above back to Main line
4. Probably delete the branch created in Step 1 (if not needed)
5. Now you will have Mainline with previous version and one branch (created in step 2) with current version
With this, you might not also lose your changes from current version if needed.
It is always advisable to work with Branches rather than directly working in Mainline.
These forum postings will walk you through replacing the main line with the contents of a branch line:
As for rolling back to a previous version of the main line, create a branch line with the tagged version of the main line you want to use and then follow the directions in the postings above to make that your main line.
Hope that helps,
For merging your branches to Main line, you can just open main line in modeller and user Merge Changes here option and select the appropriate branch.
In an ideal situation, this should go smoothly.