How do you change the oil sight glass in a BMW R1150gs?

easy way is to drill into the clear plastic window, screw in a sheet metal screw and clamp onto it with vise grips. Use a small pry bar to lever it out. don't push in too much, or you will push it into the motor cavity and lose it.

a small slide hammer works best, but not too many people have those.

insert the new one dry, no oil, glue, etc. tap it in around the perimeter gently.

same thing. don't drive it in too far or you will lose it.

the newest sight glass comes with a snap ring for the newer engines. you wont use this on your 1100-1150.