How to remove / fix recessed light trim
Almost all recessed light trim rings are held in place by spring clips that grip the interior of the recessed can. However, some of those clips have sharp prongs to better grip the can. 

You often have to use some force and wiggle the trim ring downward to overcome the grip. Sometimes, if you get a little gap, you can use a flat bladed tool, small putty knife or flat screwdriver, to locate the clip and push it towards the center as you ease the ring downward. Sometimes rotating the trim as you pull downwards helps, even though the trim is not really screwed in.

The trims routinely scrape a groove in the cans as they come out, but this does not damage the can and will be covered when the trim is replaced.

Work carefully, because some cans are not well attached, especially old work cans that may not be anchored to framing. I would strongly recommend turning off the breaker to the can before working. Turning the switch off is not enough, since there may be live hot in the can, even if the wall switch is off.

Finally, I would check, before removing the trim, to make sure that the lens does not come off by itself. If the lens were removable, it probably would require a counterclockwise twist while pushing gently upward.