How do I replace an inlet valve on a one piece wc?

I have a one piece wc, with a dual push button flush and the flush no longer operates. The wc was purchased and installed in January 2017. The water is flowing into the cistern, but straight out again. Not into the wc pan, but down the soil stack. I am of the opinion that it is the inlet valve which is at fault. I have purchased a Siamp 99T half inch brass telescopic inlet valve. I need to know how to replace it, as there is a difference between it and the one currently installed.