Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
The changing of this extension name, will require some things that may be difficult:
1. you will need to know the 'jack number' of the extension, that is, what jack number on the phone system box (the KX-TD816 that you mention) the phone station for that extension, is plugged into.
2. you will need to log in to the phone system box's administrator functions, using the system administrator's phone station, and the system box's administrator password.
If you don't have both of these, the procedure cannot work.
(Of course one could try using a default password, but if you had a previous phone system administrator who has left, did that previous person leave the needed, very much needed, phone system administrator password *****?)