Amount of authorized computers exceeded!!!

Over time I have had numerous work laptops and personal computers.

These have been made redundant and forgetting that they were used to make my iphone backups.

I authorized my new laptop today, but now I have authorized 5 computers already which is the maximum amount.

Problem is that 4 of those I do not require to be autorized any more as they were work or old PC's.

How do I reset this?

Please help.

Thanks