Check to see if the current seal is leaking. Close the door, turn the dryer on and move a piece of tissue around the door. If it is sucked in at any point, you need to replace the seal.
Buy a new seal and some nonflammable seal adhesive at an appliance store.
Pull off the old seal. If it doesn't come off easily, heat it with a hair dryer on the high setting while pulling gently.
Paint the adhesive on the back of the new seal.
Put the seal in place and press down.
Let the adhesive dry completely before running the dryer again.