=Downgrading from iTunes 11 to iTunes 10.7=
You may be able to go back with Time Machine but this may involve restoring other items too (https://discussions.apple.com/thread/4554293?answerId=20441404022#20441404022). Alternatively:
Back up your computer first, in case the unexpected happens.
Get iTunes 10.7 from http://support.apple.com/kb/DL1576 or the direct download link at: 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. The two ways I can think of to do this are:
1. Use the shareware Pacifist utility (http://www.charlessoft.com/) to install iTunes 10.7 including all associated system files. Details at http://forums.macrumors.com/showpost.php?p=16400819&postcount=6
2. Check this reference on how to delete the iTunes application itself:
Delete iTunes in Mac OS X 10.7 Lion - http://osxdaily.com/2011/09/13/delete-itunes-in-mac-os-x-10-7-lion/
After deleting the application there may be other files that need downgrading too. See the note about error -42408 at the end of this post. You may want to tuck these away somewhere safe until you have completed the installation of iTunes 10.7. I have not tested this but ideally if newer versions are not found then the installer will put in the old versions. This may include these files in /System/Library/PrivateFrameworks/ which apparently get updated by iTunes 11:
AirTrafficHost.framework
CoreFP.framework
DeviceLink.framework
iTunesAccess.framework
MobileDevice.framework
After doing one of the two procedures above 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
If you encounter error -42408:
iTunes: Advanced iTunes Store troubleshooting - http://support.apple.com/kb/TS3297 > Specific Conditions and Alert Messages: (Mac OS X / Windows) - including specific error codes.
Alternatively, check https://discussions.apple.com/thread/4556384?answerId=20441424022#20441424022 which requires you have a Time Machine backup (though possibly if you remove the newer version of these files old ones may be installed with the iTunes 10.7 installer - untested). A variant of this is at: https://discussions.apple.com/thread/4555626?answerId=20448184022#20448184022
