On some toilets there is not a significant amount of adjustment (for the water in the bowl), but try this... inside your tank there is a tube that flows into the overflow pipe, assuring that this...
If you are talking about a low water level, this usually can be adjusted by insuring the fill line from the ball-cock (fill valve) is not clogged. This runs water into the overflow tube inside your...
If you have a ball on the end of a rod in the tank, most people just bend the rod a little bit to lower the water level. If you have the fill valve that is in the corner and has a float the rides on...