Once you have loosened the fastener ,the lense should just drop off . One end is held in with a screw , the other end "Clips"
If its been there for a while it might need some gentle persuasion , it shoudnt be more than hand tight.
If you have an 1100 with an aftermarket can , it may well have melted the underside of the housing ,on the back left Exhaust side ,if thats the
case , cut around where the lens meets the black plastic.
If i have misread your question : disregard the above :