You should be able to do that right within ServiceWeb/Administrative portal.   If you are just changing which user you see on the sidecar, click Users>User List>Users with Extensions>click the user with the side car>Devices and Numbers>Presence.   The scroll to the key that you are wanting to change and click the three vertical dots on the right.   Remove a user if you are replacing and then click again and choose Assign Key.   You will get a list of users on the system that you can add.   Then press save at the bottom right hand side of the screen.   Your phone will reboot and then you should have the key changed.

I found it thanks.