Hi,
I am wondering how do I calibrate the compass. It seems that the only option for the calibration is when the unit requests it "compass error, calibrate compass".
I can't see any menu in the app for the compass calibration function.
Thanks!

Look at the screen that appears after you tap "camera" in the DJI GO app -- AIRCRAFT STATUS.  Compass is the 3rd item down.  

You can also bring it back up by tapping the top center banner.

When your Phantom tells you there is a lot of magnetic interference and lists compass calibration as one of the options, don't calibrate the compass.
You get that error message because you've put the Phantom on a manhole cover, reinforced concrete or your car roof and the compass is right on top of a lot of steel.
The correct action is to move away from the steel causing the problem.

You don't need to recalibrate the compass over again and again once you have a good compass calibration.
Read this for more info:
http://www.phantompilots.com/thr ... plete-primer.32829/

Like others have mentioned, you would want to try relocating the unit before resorting to calibrating the compass. Of course if the issue persists, then you would want to calibrate. You would follow Gil's advice as to where to find the calibration button.