The iTunes update can be downloaded from the software update function in the Mac App Store.
iTunes 12.3.1 comes just over a month after the release of iTunes 12.3, which brought support for iOS 9 and OS X El Capitan. iTunes 12, which brought a major redesign to the iTunes software, was initially released in 2014 alongside OS X Yosemite.
Article Link: Apple Releases iTunes 12.3.1 With Stability and Performance Improvements