Does anybody know where I can get instructions on how to change the dryer belt on a Frigidaire/Electrolux stackable washer/dryer model # gleh1642ds?

Here is a manual: http://dl.dropbox.com/u/8010163/Frigidaire_27in_Washer_and_%20Dryer_Gas%20and%20El ectric.pdf
If it is old enough to need a belt you need to clean it, also. Lint deposits build up like concrete in all air passages and must be scraped out and then washed out with water. You remove the pieces, first, which come out with a couple of screws. You also need to grease all bearings and support rollers with white lithium grease, available at any appliance or auto parts store. You can go on YouTube to see how to fix almost any appliance. They have very good videos used to teach you how to do almost anything. If the dryer is not very clean it will fail by blowing expensive safety fuses. Make sure to clean the exhaust hose and piece on side of house if you have one.

Where can I learn how to change a belt on a Sears stackable washer and dryer

Go to YouTube and you should be able to find a video. They are all the same, so if you know how to do one you can do them all. It's really easy to do.