First you need iTunes 12.1 which you will no longer find linked on Apple's web site since who in their right mind would prefer anything except the newest and best? Apple does not like third party sites providing iTunes downloads (and will get them removed) and you need to be leery of those anyway since there is potential for other things to be installed too. This, however, is Apple's server --> https://secure-appldnld.apple.com/itunes12/031-19002.20150409.9C24E/iTunes12.1.2 .dmg
One person said all they did was delete the new application using Terminal commands and install the old one. I wonder about this. Typically a new version will install things all over the place which only work with the newer version and when you install an older one the installer will complain or fail or at best result in all kinds of errors when you try to run iTunes.
More typically you have to use special steps and helper applications to run a complete downgrade.
Then you also need old copies of your library files which get irreversibly updated when opened by a newer iTunes.
A general web search for something such as "iTunes downgrade" will uncover detailed instructions.