There's no certain way for me to know which fill valve you have in there, but locate the float. This floats on the water level and turns off the water when the tank is full. There's probably a screw on the float arm that lets you adjust the float up and down, adjusting the full water level. You may be able to loosen a collar on the bottom of the fill assembly and slide the whole thing up and down (make sure you turn off the water valve when you do this because the whole thing can pull apart), or you may be able to simply turn the entire fill assembly which will raise and lower it if it's built that way. Make sure the new water level doesn't go above the level of the refill tube or the water will go down the tube and the valve will never turn off.
Answers & Comments
There's no certain way for me to know which fill valve you have in there, but locate the float. This floats on the water level and turns off the water when the tank is full. There's probably a screw on the float arm that lets you adjust the float up and down, adjusting the full water level. You may be able to loosen a collar on the bottom of the fill assembly and slide the whole thing up and down (make sure you turn off the water valve when you do this because the whole thing can pull apart), or you may be able to simply turn the entire fill assembly which will raise and lower it if it's built that way. Make sure the new water level doesn't go above the level of the refill tube or the water will go down the tube and the valve will never turn off.