Is it internal or externally glazed? if it's external then there should be fairly thick rubber seals al round. use a sharp knife or similar to carefully prize one end of a pice out, then pull the whole strip out. Once one is out the rest are easy, but getting the first out is tough if it was fitted properly. To add refitting coatwith something like fairy to 'lube' it, push one end in and work along. really push it in...it will try to stretch and if you don't work it in you'll beleft with a few inches over....don't be tempted t trim off, pull out and work it in harder.
If it is internally beaded then the plastic beading and rubber seal usually come off in one. Again sharp knife or wallpaper striper, carefully lever one end out then pull, shold just unclip basically. refit is as before, clip one end then work along.
Unit itself is probably sat in the frame on a few spacers, take note and refit the same way.