How do I go back to iTunes 10.5?

You have to uninstall iTunes then install the old version

iTunes 10.7 download

64 bit

http://support.apple.com/kb/DL1577

32 bit

http://support.apple.com/kb/DL1576

You also need to recover the backup library made when you updated.

http://support.apple.com/kb/TS1967?viewlocale=en_US

If there have been a lot of changes to your library since updating you may prefer this:

http://support.apple.com/kb/HT1451

Your current library is recreated from the xml file, but all the date added fields will be the current date.