Q: DeAuthorizing Computers

I have an iPhone 4. I have had to borrow pc's of friends over the years to do any updating and file transfers on my phone.

I have attained a re-furbished laptop of my own now and cannot Authorize my phone on it because it says I've reached my alloted 5 pc max.

I am instructed to de-Authorize the other computers first. Alas, I have no access to those computers. How can I de-Authorize them at this point?

thanks H