How can I authorize or deauthorize a computer in iTunes?
To authorize a computer, install iTunes and then go to Store > Authorize this computer. Enter your Apple ID and password and that's it. On Windows press Ctrl + B to see the menu. You can use maximum 5 authorizations.
To deauthorize a computer, open iTunes and go to Account > Deauthorize this computer.
There are in-depth guides for these operations. I suggest you read them for possible issues that might be encountered.