Your device was already calibrated at the factory, and the device uses automatic calibration by default. You can manually calibrate the barometric altimeter if you know the correct elevation or the correct barometric pressure.
1. Go to a location where the elevation or barometric pressure is known.
2. Select Elevation Plot.
3. Tap on the three white lines (bottom right), and select > Altimeter SetUp> and then Calibrate Altimeter.
4. Follow the on-screen instructions.
For this to work at its best the GPS really needs to be on and have a satellite fix for 20 - 30 minutes so it can work out the correct barometric pressure.