--IMPORTANT!-- If you have decided to revert from version 12 to version 11, you must be sure that every computer running version 12 is put back onto version 11. You cannot have multiple computers accessing the same data where some computers are using version 11 while others use version 12.
--CAUTION-- If you revert to version 11, you will have to restore the most recent version 11 backup that you made before upgrading to version 12. Whatever data you entered after moving up to version 12 will be lost and will have to be re-entered. You can find the steps to restore a backup here.
If your most recent version 11 backup is extremely old, causing you to lose more data than you are willing to lose, your only option is to stick with version 12 but it will have to be reinstalled on a Windows 7 computer or a Windows 7 VM.
--IMPORTANT!-- If you decide to revert to version 11, you must first do the following...
1. Go into Services and stop Firebird 3.0.
2. Open the Control Panel > Programs and Features to uninstall Firebird if you see it listed.
3. Go to the following three places and delete the firebird folder if you see it.
- C:\Program Files
- C:\Program Files (x86)
4. Install version 11.