There is a screw on the fill valve on the left side of tank and you will turn it counter-clockwise to lower the water level in the tank... if this doesn't work, you'll need to replace the fill valve. Also the flapper or seat gasket (depending on which flush valve your model uses) may need to be replaced. This is often times noticeable by the tank filling up every once in a while without a flush. But if it constantly runs, you need the fill valve.