DJI- sent me a replacement for my phantom3se.  Do I need to register this new one or did DJI- DO that before shipping it to me?  If so how do I go about doing that?

When you first register a drone, the same FAA number applies to all drones you buy afterward.

Try it and see.
A Phantom won't fly unless activated.
If it flies, it's been activated.
If not it will prompt you to.

It flies fine so far.  Thanks

Unless your registering under part 107. Then you’ll wil need a registration for each aircraft. Recreational pilots basically register themselves, 107 pilots register each aircraft.