Shut off the filtration system and pump to the pool.
Find the sewer drain port of the pool, which is located near the main line and has a plastic cap with a square on top. Open the port by placing a wrench around the square part and turning it counterclockwise.
Connect a drain hose from a submersible pump to the sewer port and place the submersible pump into the pool.
Turn on the submersible pump and drain 2 to 3 feet of water out of the pool so that it will be easier to stretch the pool liner back into the track. Turn the pump off when the water level has lowered.
Plug in a heat gun and hold it 12 to 18 inches away from the area of the liner that has popped out of the track. Hold the gun in place for approximately five seconds. Turn the heat gun off and set it aside.
Grasp the top edge of the pool liner that is loose and slowly stretch it backward until you can insert the top of the liner back into the track on the upper edge of the pool. You may need to apply additional heat to accomplish this.