=Downgrading from iTunes 11 to iTunes 10.7=
You may be able to go back with Time Machine but this may require a complete restore of your computer (https://discussions.apple.com/thread/4554293?answerId=20441404022#20441404022). Alternatively:
Back up your computer first, in case the unexpected happens.
Get iTunes 10.7. Apple does not keep many old version up on its web site. I always install iTunes by downloading the installer and then keep it on my archive drive so I can have access to old versions. This one is no longer openly linked on Apple's site:
iTunes 10.7 for OS X http://appldnld.apple.com/iTunes10/041-7195.20120912.d3uzQ/iTunes10.7.dmg
Replace the iTunes 11 application with iTunes 10.7. Lion (OSX 10.7) and newer systems have iTunes integrated into the operating system and deleting is a bit more involved. Check this reference on how to do this:
Delete iTunes in Mac OS X 10.7 Lion - http://osxdaily.com/2011/09/13/delete-itunes-in-mac-os-x-10-7-lion/
Now you will have to rescue your old iTunes library from your iTunes > Previous Libraries folder and rename it "iTunes Library.itl" (replacing the existing one in the iTunes folder) because a newer version of iTunes irreversibly updates your library file. Note, this will revert your library to version at the time of the upgrade and you will have to update any changes made since. See:
https://discussions.apple.com/thread/4544824?answerId=20401436022#20401436022 - turingtest2 11/2012 post on rebuilding empty/corrupt library from previous iTunes library file after upgrade/crash.
iTunes: How to re-create your iTunes library and playlists - http://support.apple.com/kb/ht1451
Other issues:
https://discussions.apple.com/thread/4553961?answerId=20432309022#20432309022 - solution to mobile devices saying they need to be restored after downgrading