Inlet needs adjusting to reduce water level and cause the valve to be shut earlier than currently is.
If the upended cup type float rising up a ratchet type rod the adjuster is the long rod down to bottom [almost] of cistern this needs to be adjusted up or down to reduce level.
If the Ball float and arm type then adjustment is at the inlet valve end. The threaded road meets a plunger rod in the valve to shut water off.
Release the locknut and turn the small threaded rod, by It's screwdriver head, in towards the inlet valve plunger. [about 3 full turns] then allow re-filling and check the level it stops at, best about 25mm lower than the level mark in the cistern but let it run higher if then it's doesn't flush properly. When right re-lock nut
Nobby
December 2008