as said if your sticking with the same type of flush valve then they seperate half way down by twisting the top part 1/4 of a turn.
if changing completye valve then the cistern will need removing.
these type of flush valves useally never need changing
only the seals at the bottom fail over time
and all are replaceable for a few quid.
hi-spec plumbing & heating (essex)
March 2010