Since your tank has set for a few days, the water should either be room temperature, or else at whatever temp you set your heater at. Float your betta in his container for 20 minutes. This allows the water temp outside the bag to equalize the temp inside the bag. This also provides time for your betta to chill out before being put into a new enviroment.
When putting the betta in the tank, open the bag, pour it into a net suspended over a bucket (to catch the water). Once netted, gently place him in the tank and let him swim out. You don’t want the water that he came in to be in the aquarium, as its often dirty and can carry diseases.
If there are other occupants in the tank, before releasing the betta give them a touch of food to distract them. You don’t want them swarming your betta as he enters his new home!!!
He may not eat for a day or so. Don’t worry, he will eat eventually!