The brightness of the photo makes it hard to tell what the damage is, but if it's just the rubber trim getting rough, sometimes it's just breaking down from UV rays and weather, and nothing can stop it.
If it's just the surface getting flaky and chunking, you might try some high grit sandpaper (use some regular paper or thin cardboard and tape on the window/paint to keep it from getting scratched) to remove the stuff on the surface, then polish it smooth with even higher grit sandpaper and/or some rubbing compound.
Liquid or gel products that claim to treat and protect rubber actually seem to have the opposite effect - Armor All and the like eventually cracks and rots rubber, vinyl, and plastic, so I'd advise against those.
If the trim is so bad you want to replace it, you'd probably have to do so by getting an entire window, since the studs are part of the window and the trim. It may be possible to seperate them, but it might break the window.
If you do replace the window, you'll need to be careful pressing it out since it's sealed and held in with adhesive in addition to the studs. You'll of course need new adhesive to stick another window back on.
