how to replace a new valve in a 3way valve?

have just purchased a diverter valve diaghragm repair kit for my wicks 3 way valve.
Can anyone tell me how to replace it. I know to turn off all electrics & stop the water filler in loft
Do i need to drain all the top underfloor pipes so the valve has no water in it.
Do I need to remove the 3 way valve completely out off the pipes to do the repairs.

teresa gardner
March 2015