How do I change administrators?

How do I change administrators?

I am trying to change the administrators on the account.  The person listed is no longer with the company, and I can't change the email addresses available on the Edit Company Details because it's only a dropdown box.  I went to a different page to try to fix it, but there is no option to deactivate the employee who is no longer with the company, nor change the email addresses to current valid ones.  How do I do this?