Cynthia, the valve is not an overflow valve, it is a pressure relief vale. Why you need it is that if the burner does not shut off, the water in the tank can boil, turn to steam, build up a great pressure, and you can have a very large EXPLOSION.
For this reason, the valves are set at the factory and can not and should not be adjusted.
There are 2 possible explainations. One is that your temperature is set too high or getting too high. This is dangerous. The second, and more probable, is that the valve is bad. Turn down the water temperature. If the dripping does not stop, replace the valve. If it still doesn't stop, call a professional service person. This might be under warrantee, and so might the valve replacement. You really need to get this done and not just because of the cost of the water.