How do we change the flush from 1/2 to a full flush?

Yours is obviously a single flush (unusual) most cisterns have two buttons, half and full.
Seems that the cut off valve operated by the float is shutting off to early. Inside the side of the cistern there should be a water level marker(a line) bring water level to that line by altering the float cut off there should be an adjustable threaded rod somewhere on the float arm level with the inlet valve.

bob
August 2013