Open fireplace door slowly so you don't scare the animal. Sit back and if all goes as planned the squirrel should run toward the light coming from open windows or doors and escape. Try following it and directing it to the outdoors or capture it with a blanket if possible and quickly carry it outside in the blanket. The squirrel may get tangled in netting. Never grab a squirrel, even if you have gloves on.
Squirrels can bite through any glove.
Then you will have a smelly dead animal to remove. You could break through the floor and fall into the room below. Enter the attic and make a lot of noise to scare off an animal. Set up a radio with a talk station on and turn it up loud. Set up a bright light, but make sure the light is not a fire hazard.