How to Deauthorize a Computer in iTunes
Apple puts a limit on how many computers you can use with some of your owned iTunes and App Store content, including music, movies, apps, TV shows, books, iBooks, this process is known as iTunes authorization. Most users don’t pay much attention to this, but if you own multiple Macs or PCs concurrently or over the years, you’ll eventually hit the 5 computer authorization limit in iTunes, often when trying to access iTunes content or restore an iPhone, which then prevents you from accessing that iTunes Store and App Store content until another computer has been deauthorized, and then the current computer authorized.
The solution to this is to deauthorize a computer from iTunes, a process which can be necessary in both Mac OS and Windows.