How can I increase my weight and gain muscle?

With hard work! First get clearance from your family physician or geriatrician, then start exercising. There's no day like today to start! you don't need to go to the gym or buy any fancy equipment. Just try a few push ups, squats and a short walk to start. You can also curl & press cans of food and graduate up to gallons of water. Don't forget proper nutrition w/plenty of lean protein, fiber & healthy fats.