there is on most cisterns adjuster on the water inlet valve/float depending on type to reduce the water level. But don't reduce too much or you may find the water is not enough to flush out the toilet solids, and it could then get very Stinky poo!
on the ball and arm type the adjuster screw is at the valve end secured by a lock nut and it's plastic. Slacken off the nut and turn the threaded plunder rod in the direction that extends towards the valve [clockwise] check how far it fills if satified re-tighten lock nut. I not this type then you'll have to search for adjustment or use a brick like previous replier said.
Bob
October 2010