Change name of Mail user

An office computer was set up with an email address and a specific employee's name. That person is no longer employed here. We want to keep the email address (and previous messages) but want to change the address owner's name. 

TWICE now I have walked through the process of requesting a change to the account, and received an email with a code. BOTH TIMES I have used the code provided and received a message "The code didn't work." What am I supposed to do?