Hold UP, and select Settings > Sensors & Accessories > Barometer.

Calibrate

Allows you to manually calibrate the barometer sensor.

Plot

Sets the time scale for the chart in the barometer widget.

Storm Alert

Sets the rate of barometric pressure change that triggers a storm alert.

Watch Mode

Sets the sensor used in watch mode. The Auto option uses both the altimeter and barometer according to your movement. You can use the Altimeter option when your activity involves changes in altitude, or the Barometer option when your activity does not involve changes in altitude.

Pressure

Sets how the device displays pressure data.