The altitude on an altimeter watch is based on atmospheric pressure. This means it is sensitive to changes in weather, so you'll need to calibrate your device regularly.

To do so, proceed as follows:

1. Starting from the "time" screen, press the MODE button (bottom left)
   
   
2. Once in ALTI mode (altitude is displayed), hold down the SETTINGS button (top left) for 3 seconds
   
   
3. The altitude will start flashing
   
   
4. Adjust to the altitude of your current location (obtained from a map or elevation marker) by pressing the buttons on the top right (+) or bottom right (-)
   
   
5. Once you've set the altitude, exit the menu by pressing the SETTINGS button (top left)