Hold MENU, and select Settings > Sensors & Accessories > Altimeter.

Barometer

Allows you to set the barometric pressure manually, or select a value from the nearest METAR station through the Garmin Connect™ Mobile app.

Altitude Alert

Sets an alarm to vibrate when you reach a specified altitude.

O2 Alert

Sets an alarm to vibrate when you reach a low-oxygen altitude.

Sync to Elevation

Allows you to synchronize the device to your current elevation manually.