Changing the transfer case oil is really easy to do. You'll need a liter of BMW transfer case fluid, and we recommend two new sealing washers (rings) at the drain and fill plugs.
Transfer case Fluid, BMW p/n: TF 0870 83220397244 (83 22 0 397 244)
Sealing ring, BMW p/n: 07119963308 (07 11 9 963 308)
It is also a good idea to reset wear adaptions that are stored in the Xdrive control module, especially if the you have a VTG oil wear fault code (54C6) stored. This requires a BMW diagnostic scan tool to reset.