iPhone 6s back popped off

The back is attached to the screen with a frame and adhesive, your screen is coming up, There is two pentalobe screws on either side of the charging port, verify those are still screwed in. You will need to unscrew those in order to remove the screen. It will fold up, but do not try to pull the screen entirely up, as there is important cables attached to the logic board. There is two notches in the plastic frame that need to go into the aluminum frame before you can set the screen back into the rest of the frame. Using your fingers on the edge of the frame and body, push gently until it snaps back in. Once you have the screen secured into the body, you can screw the two pentalobe screw back in. If there is any damage to the body such as: Bending, warping, or enough scuffing, the screen may not fit properly back in.