is it the sliding unit? If so, remove the felt weather strip and then slide the opening unit back and remove the felt in there aswell. You will now see the rivets that hold the frame to the body. drill the head of the rivet off and carefully (remember the Haynes translation for carefully lol) pull the frame away from the body. fitting is the reverse but you will need a thin extension for the rivet gun as the normal head is to fat to get into the channels.
If the glass you are talking about is the fixed, single pane, then it should be fitted into claytonrite rubber which is a "H" section glazing rubber fith a filler strip. simply remove the filler strip by hooking a dibber (ground smooth flat blade screwdriver) into the join. Then run the dibber inbetween the glass and the rubber to break the seal of crud and sealant that has built up over the years. To remove the glass from the rubber there are 2 methods.
1) using a 50kg APAL lifter (glass sucker) fit it to the outside of the glass and pull on it at the same time as you run the dibber between glass and rubber. remember, the glass is toughened glass and is weaker at the edges and may shatter.
2) get someone to push on the glass from the inside while you run the dibber between glass and rubber
in both cases, work fron a top corner, get the top free of the rubber then the sides then lift the glass up and clear of the rubber.
The only other way is to cut the rubber with a stanley knife and fit new rubbers when re-fitting the glass but it is expensive and usually leaks