I have a 2006 K1200S.
My drain plug is straight on the back of the final drive, pointing towards the back. I replaced the stock drain plug with a magnetic plug.
Remove the rear wheel and put the bike on the center stand. Remove the torx bolt on the stabilizer bar at the top of the final drive.
Work the flexible boot loose and tilt the final drive down. The drive shaft slides off the splines so just work it off the splines as you tilt the final drive down.
Remove the drain plug and let it drain for a while. The fill bolt is behind the brake disc accessed from the right side. You will need to turn the disc to position it so you can get to it.
On my bike the fill plug is the sensor. I remove the sensor and using a syringe but the new oil in there. They have changed the recommended amount for the oil/fluid. Use Synthetic GL-5 75W-90 6.087 oz
. This cuts down on the pressure build up inside the final drive and saves the seals.
While the oil is draining, use a couple of screw drivers and carefully work the boot back enough to revel the drive shaft and splines at the back of the engine. Work the drive shaft toward the back of the bike exposing the splines. I use a small screw driver like a spatula and spread the lube on the splines. With the transmission in neutral, rotate the drive shaft and paste the lube on the splines. Now go back to the final drive and paste the lube on the final drive splines.
Put the drain plug back in and using a syringe put in the new oil. Install the plug or sensor depending on your bike. Make sure you have the boot cover in place. Now align the drive shaft with the splines on the final drive and start tilting the final drive back up working the drive shaft back onto the splines. Work the boot back into place and when you get everything lined back up put the torx bolt back in and torque it to specs.
Now go back up to engine where the drive shaft attaches and work that boot back into place.
I change my oil every winter. I believe the spec is every 12,000 miles. There are others on this forum that have additional ways to help out.
Oh, check this out: