Instructions
Switch your watch to "Altimeter" mode. Press the "S3" button, located at the bottom left of the watch's face.
Press the "S1" or "S2" button to toggle through the Altimeter menu's option--both buttons are located on the right side of the watch. Stop when "SET ALTI" appears on the watches' display.
Use the "S1" and "S2" button to raise or lower each value in the current altitude, and enter the new value.
Press the "S4" button to save your altitude change.