How do I deactivate 5 computers, which were actual Hard Drives replaced, and activate just this one?

How do I deactivate 5 computers, which were actual Hard Drives replaced, and activate just this one? I could not solve this puzzle around 8 years ago, and now that I am attempting to use my iPod Classic again I remember just why I left everything alone and walked away from Apple iTunes. I sure could use some help to get around this mess! I think I explained enough?! I thank anyone in advance for any real suggestions... I have an iPod Classic 160 GB and YES, it still works and runs just fine. Problem is having Apple work with me instead of against me and my account.