The furnace in our home is GM # 80904 and don't know how to change the Filter or where its at.

The filter can be anywhere in a return system. Look at the furnace and see if there is a metal handle where the air return meets the furnace. Filters are usually there. Pull the handle out and hopefully a filter is there. If no handle, look for a large wall return, 24X24 or so. You may see small metal tabs on the bottom. Move the tabs over and lift the front, the filter should be in there. Last, look to see if the individual returns have filters in them. Some home builders put them in each room, easy to replace but expensive.