how do I replace a pressure reducing valve in a worcester 24i boiler ?

My boiler is running out of pressure, there is no leak anywhere, as soon as I refill the boiler, the pressure gauge drops straight back down within two minutes, there is hot water coming out of a pipe from the boiler outside wall, although there is no pressure in the boiler it is working ok central heating and hot water. Any advice would be greatly appreciated.

Cheers Tony