You unscrew the 4 sheet metal screws holding it to the box. The brass tubes have o'rings on them, Once the four screws are out, you just pull the copper tubes out. They can be hung up from age, but that's all that holds them in. Heating them up destroyed the sealing rings. New ones come in the rebuild it. The H valve inside uses rubber hoses to connect the heater core to the H valve. What you are doing is then pressing the copper tubes into the H valve, sealed by ORings. But that's all internal, it will make sense once you open it up.