The leak detector will remain in an alarm state as long as it is detecting moisture. To stop the alert, you will need to make sure the device is thoroughly dry. This can even include environmental factors, such as moisture from your hands or humidity in the environment.
If drying the device does not stop the alert, there may even be trapped moisture inside. Using a screwdriver or a coin in the blue edge around the outside seam for leverage, open the sensor and remove the batteries. Place in a cool, dry environment for several hours before reinserting the batteries and replacing the sensor.
Continued false alerts may mean that its current environment is not a good fit for the sensor.