First, please separate the tracker from the bottle. Once separated, the battery compartment on the tracker should be clearly visible.
To replace the battery simply tilt the tracker and lift the positive (+) pin holding the battery only to the extent necessary so that the battery comes out. Insert the new battery with the positive terminal looking upward.
(Please make sure not to pull on the positive with too much force as that may deform it. That could decrease the chances for the pin to have good surface contact with the battery. Good surface contact is necessary so that the battery can power the device.)
While replacing the battery, please make sure you don't excessively lift the positive pin holding down the battery. This may make it so that the new battery is not tightly held and if the connection between the terminals and the battery isn't firm and secure it may cause the H2OPal to turn on and off or malfunction.
The battery model that the H2OPal uses is the CR2450.It's available in many stores as well as online. Here's an example listing for this battery on Amazon: