Unfortunately, you can't just drop iTunes 11 on top of 12 as it won't read the library files. I don't know if there are data changes or merely a version check, but 11 will refuse to open a library created or modified by 12. In my case, I have a recent backup from 11 I was able to restore. Note that doing this will cause amnesia on any recent additions or changes to the library. Exporting the current database may do away with that issue, I didn't check.
- Export the library via File \ Library \ Export
- Uninstall iTunes 12 via Programs And Features in the Control Panel, reboot
- Download and install your preferred prior version via Old and New Version of iTunes Download - OldApps.com
- Import the backed-up library via File \ Library \ Import