There is no need to de-register the handset as you can register up to 5 handsets and old T-Hub handsets are compatible with T-Hub 2.
But just in case you wanted to, Here are the instructions on how to unregister a handset on the T- Hub.
Select “Settings” from the idle screen then [Ok] Scroll to “Base Settings” then [Ok]. Scroll to “Delete Handset” and then [Ok] You will then have the ability to unregister any of the Handsets registered to the Base Station. Select the name of the handset that is to be deleted and select [OK] You will then be presented with the message ‘Do you want to unregister this handset?’. Select [No] or [Yes] By selecting [Yes] you will be presented a message ‘ Handset unregistered from the base’. Alternatively you will be taken back to the idle screen.