There is no special transmission fluid per se. The transmission is lubricated by the engine oil.
I guess you are asking about the final drive in the rear. The method for changing the final drive fluid varies, depending on the model year. The early slant 4 models (2006) had "sealed" final drives with no drain plugs. I think you remove the final drive fluid through the abs sensor hole.
I suggest that you do searches for "final drive oil" and "final drive fluid" on k-bikes.com and i-bmw.com. You should find several threads and how-to's there. There are also great maintenance how-to's on the BMWMOA website.
Another option is to go to your BMW dealer and order the "REPRom" repair DVD. It has all the repair and maintenance procedures for your K1200R. It isn't cheap (around $100) but it is helpful. Please don't be tempted by the illegal stolen bootleg copies from the Internet.
(Additional note: You are posting on the KGT forums, but you are asking about a K1200R.)
Good luck!