They are just pushed into place. just pull out the old gasket, it may be stuck at the top in the middle, just wrench it out and clean out groove the best you can. Then ease in new gasket, bought from any double glazing firm, with your finger, starting at the top, once you are back at the top, place a small amount of super glue on the end and push both ends together. DO NOT STRETCH the gasket while inserting.
Burt
November 2009