OK
There is a drain plug and a level / fill plug on the gearbox.
Drop bottom cover on front diff to drain, clean mating surfaces, then refit using silicon sealer. Refill through fill / level plug.
On the rear diff, you will need to support the diff with a jack, unbolt the mounting, remove the cover bolts, then loosen and slide the cover down and off to let the oil out. Again, clean the mating surfaces before reassembly using silicon sealer. Refill through the fill / level plug.