990R dash is the same as the 690, you need to cut a wire to the back of the dash that locks the wheel size. You can then adjust wheel size using the mode and set buttons. I did it on my 690 and worked well.
These are the notes I kept, this is the procedure once you have cut the wire:
*Turn key to "on" and with the meter set to "Trip 1", press and hold "Mode" for 10 sec.
Length xxxx (the 17" circumference) will show up (flashing), press "mode" to increase and "set" to decrease the circumference.
The correct setting for 21" should be 2205.
*Press "mode" and "set" simultanously to set it.
You will need to search the big 690 thread the find which wire to cut, I recall there are pics.
KTM 990 Adventure S