To stop the leak detector from sounding, you will need to make sure that the device is thoroughly dry. The leak detector is very sensitive to moisture, including any moisture from your hand if you are touching the silver rings along the bottom. It will beep as long as it detects moisture.
Often times after a water leak is detected the device will have unseen moisture inside. If you have dried the entire outside of the device and it still continues to beep, please open it up and remove the batteries. Leaving it in a warm dry area for several hours before replacing the batteries and putting it back together.
The device opens in half along the blue edge. It is designed to be a very tight fit so a flathead screw driver or a coin is helpful in popping it open along the edge, much like you would a paint can.