Roll back from OS X Mavericks ( or any newer OS) to Mountain Lion
Since the question does not mention backups, I guess there aren't any.  What you can try in this case, is to make a time machine backup (e.g. on an external USB Harddrive) of the current Mavericks partition. 

Then do a clean install (as suggested by jaume) and then use the migration assistant getting your data back from the Maverick backup into Mountain Lion.