Changed Apple ID, but can't update it on iMac

Hello,

My apple id was created before I had any apple products (using itunes on pc), and I just used a spam email account. Years later, I wanted to update it to an appropriate email address, since I now have an iphone, imac, ipad, etc.

With a little magic and curse words, I got it changed successfully on my iphone and ipad. But I am unable to get it to change on my iMac. I get notifications often that I need to enter the password, but I am powerless to make it work.

I have Yosemite 10.10.3.

On the iCloud panel I am trying to "sign out" and delete the account - and that goes okay until it wants a password to turn off "Find my Mac" - I've tried both old and new passwords, but nothing works. I tried logging in to the icloud account online and going to the "find my" section, but there is no option for removing the computer from the find tool.

What can I do to update the apple id on my computer?

Thanks!