My recommendation is don't even think about it. It is not designed to be a service item and unless you have done a zillion gear changes over a zillion miles it does not need changing.
If you really insist it is a matter of removing the the carpets, removing the gear knob, removing the leather cover off the gearbox cover, removing the seats (first unplug the seat belt sensor wire) , peeling the leather off the transmission tunnel (it is glued), removing the transmission tunnel (Morgan in their wisdom fitted it over the back of the gearbox case) .
Then the fun starts! Remove all the screws on the gearbox cover, then struggle and swear to free the mastic that effectively glues it in place. That is when you discover that you can't actually remove the gearbox cover because the bottom of the heater air box is in the road. At the factory I understand that they remove the heater box. You may be lucky and be able to force it off by lifting at the back and pulling and risking a bit of damage. I couldn't get mine off. With the cover propped up and arms like an octopus you can then remove the remote, if a Plus 4, or whatever a 4/4 uses. You can then expect a bit of swearing as you clean the spilt oil off the carpet that is fixed in the under seat area.
Having done all that you can faff around with the oil of your choice and then reverse the procedure. Oh! Don't forget that you will need new gaskets.
I know all this because I had a gearbox problem that eventually required removal of the entire gearbox.
I don't anticipate ever changing the oil in the top of the gearbox. After my experience, at the very most, I would only do it if the gearbox has to come out.
All I can say if you decide to do it is "Good luck and keep smiling."
PS. It is a bl**dy awful bit of design that could be fixed with 2 minutes of thought and a bit of tha very rare common sense