How To Adjust the Fill Level
If your toilet bowl is not flushing completely clean, you may need to increase the water fill level in the tank. If you wish to conserve water, you can also adjust the fill level use less water in each refill cycle.
After pressing the flush handle, the tank empties most of its water into the bowl then the tank begins to refill. It stops automatically when it reaches a preset level. In nearly all cases, that level is controlled by a float that is lifted by the rising water. When the float gets high enough it closes the water valve.
In order to adjust the water level, you must set the height at which the float shuts off the water valve. There are several styles of floats. In older toilets, the float hangs at the end of an armature, along a metal or plastic rod. Where that arm connects to the water valve there is usually a screw. Adjusting that screw will change the level at which the water valve shuts off. If there is no screw or it is already at its maximum, you may be able to bend the metal arm, but you should remove it from the valve first or you risk damaging the fill valve.
In newer toilets the float is often a collar around the fill valve and rides up and down vertically. The float typically has a spring clip connecting it to a metal rod which connects to the valve shut off control. Squeeze the spring clip to slide the float up or down the metal rod.
Each manufacturer may have variations on the fill valve mechanism, but they all include a method for adjusting the water level. Close inspection should make it clear how to adjust your particular fill valve.