Well if it is the same as the 720, you will find a small recess in the leading edge of the glass lens/cover. Using a blunt screwdriver, insert it in the slot and twist it. The lens should gradually come away from the side of the oven. DO NOT attempt to do this if the oven has been in use as the lenses will become too hot to handle. Put a cloth on the bottom of the oven to catch the lens as it falls to protect it. Note that there may be a cut out on the inside of the lens to go over the edge of the bulb/bulb holder, so don't try to put the lense back round the wrong way.
Answers & Comments
Well if it is the same as the 720, you will find a small recess in the leading edge of the glass lens/cover. Using a blunt screwdriver, insert it in the slot and twist it. The lens should gradually come away from the side of the oven. DO NOT attempt to do this if the oven has been in use as the lenses will become too hot to handle. Put a cloth on the bottom of the oven to catch the lens as it falls to protect it. Note that there may be a cut out on the inside of the lens to go over the edge of the bulb/bulb holder, so don't try to put the lense back round the wrong way.