This job could turn into a heater box out job!! The two small bolts holding the valve go to a captive plate boss (nut), it can drop down and the only thing you can do is weld it onto a wire and loosen the whole heater or remove it altogether.Take one bolt out loosen the other turn the valve and put a long holding bolt in before removing The other bolt from the valve holding plate, loose assemble in reverse to fit new valve.If it drops it will be more than beer money required to put it right.