Getting the lens separate from the holder is the tricky part. If you are very lucky you can just unscrew it but before you try look into the holder from the back to see if the lens has melted or deformed from the heat of the bulb over the last 30 years or so. Most have deformed from heat and will not budge. New ones
are available for $49.95 each. I took a small screwdriver and heated the tip and plunged it into the backside of the plastic forming a slot. Let it cool. Then used a propane torch to ever so gently heat the aluminum holder while using the screwdriver to unscrew the plastic. IT DOES NOT TAKE MUCH HEAT