The phone back cover removed

If you want to glue it back, buy a b-7000 glue. clean with some alcohol on a fiber the old glue, place a small amount of b-7000 on the perimeter (try to follow the old glue path and not cover the cables because on future repair the next technician may lift them and break them with the glue) , then place some rubberbands to hold it in place and let it dry for at least two hours.