deauthorize computers

You can see the instructions on how to do this at Authorize or deauthorize your computer for iTunes Store purchases

Scroll down that page and you will see

Deauthorize a computer

1. On a Mac, open the Music app, Apple TV app, or Apple Books app. On a PC, open iTunes for Windows.
2. From the menu bar at the top of your computer screen, choose Account > Authorizations > Deauthorize This Computer.
3. Sign in with your Apple ID.
4. Choose Deauthorize.

If you run out of authorizations, deauthorize all

1. Open iTunes on PC
2. Select "Store"
3. Go to "Music Quick Links" on the right hand side of page
4. Select "Account"
5. Scroll down to "Computer Authorizations"
6. Select "Deauthorize All"

I hope this helps and is more clear. :)

(NOTE: YOU CAN DO THIS WITHOUT THE CURRENT COMPUTER YOU ARE USING BEING AUTHORIZED)