1. To replace the battery on Tempdrop, you'll want to use a nail or plastic tool, not a metal tool like a screwdriver.
You can find one such tool on Amazon by searching for an "opening tool kit." See below.
Gently use the tool to separate the front an back sides of the Tempdrop sensor.
2. Before you reassemble the Tempdrop sensor, make sure that the frame is snapped to the bottom shell via the two plastic teeth.
Tempdrop takes a CR2032 3V lithium coin cell battery. The battery labeling should face the top shell of the sensor.
3. In order to close the sensor, hold the wider, bottom part as shown and use your other hand to snap the two shells together along the parting line. You should hear a click.