Buy a screen seal, cut it to fit just the loose part, slide it in and use a heat gun. (Of course, if you overheat the screen, you ruin it, and if you don't heat it enough it's not actually fastened, and the difference between those temperatures isn't much - which is why I don't recommend doing it yourself. It's cheaper to pay a decent shop to do it for you than to have to pay them to replace the screen.)
(In a repair shop, it normally takes a good tech about 5 or 6 ruined phones before he gets it right - which is why I never let a new tech touch a customer's phone until he had replaced the screen on at least 2 totally shot phones successfully. More than 10 tries, and some people just can't get it right, and I'd ask him to find a job in a different industry.)