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When you unscrew the light from the wall mount, there should be enough cord to place the light fixture onto the pool deck.
There may be a junction box standing up several inches above the ground straight the behind the light. It’s als possible for the wire to run all the way to the switch if it’s close. You’ll have to figure it out. If there is no junction box, look at the switch and see if there is a jacketed cable rather than all romex in the box.
Open the box and you can measure voltage to see if power and the switch are good.
To replace the whole light (fixture and all, not just the bulb):
Check to see that the cord moves freely between the junction box (or switch) and the light, but don’t pull it out yet.
Cut the cord at the light and connect the wires from the old cord to the new cord. Tape thoroughly. Use the old cord to pull the new one to the junction box.