How do I get the driver side door covering off so that I can get to glass? on 1999 GMC 1500 Pickup

The glass on the driver's side has fallen down insid3e of door.

I would not suggest you try a DIY job on your drivers door. The inside of that door is one of the most difficult to work on that GM have ever made. If you decide to remove the door panel and have a look inside I would suggest you get a service manual with illustrated instructions for removing the door panel and servicing the window glass.
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If you have power windows, you will have to take the panel with the power window buttons on it off. There are a couple of screws under it. Take the shroud around the door handle off. More screws. Then you will have to push up on the whole door panel. It's kinda tuff to do, but once you get it started, it comes right off.