How do i unauthorise a pc?

[Langtang]

In my apple account it says I have used 4 of my 5 PC accounts, but I only have a PC and a laptop. I had older pcs but don't have them anymore. How do I remove them from my total machines available?

 

[DontUnderstandMyIpad]

In iTunes, there is an option to deauthorize all computers at once, but it has a cooldown of 365 days, so only use it, if necessary.

The option should be found under Store.