The water level in a toilet bowl is adjusted using the fill valve. The fill valve determines the water level by controlling the position of the float. Raising the float arm increases the amount of water in the bowl while lowering it decreases the level of water.
The float arm may be adjusted up or down using the adjustment screw usually found on top of the fill valve. Some older toilets have no valve and may need upgrading in order to make them adjustable. Float arms come in either plastic or metal. Metallic float arms can be adjusted by bending them up or down to reach the required level.