When I fly, the radar on the bottom left isn't accurate. The arrow isn't showing forward, it shows it's backwards. How can I fix this? There is no problem with flight, just the radar isn't right.

Is the arrow on the map also wrong?  If not then it'll be to do with the compass in your phone/tablet rather than the Phantom.

I put 'flight mode' on my phone. Could that be the problem? Flight mode disables the gps and everything to the phone.

It shouldn't make any difference as it's the phone's compass that's used to determine the direction it's pointing.  Do you have a compass app on the phone?  Just check that's correct when it's attached to the controller & switched on.  Phone compasses can also be affected by interference, just the same as the one in the drone.

Please try  compass calibration on both the aircraft and your device .

Thank you guys. It worked. Had to calibration the drones compass plus my cell phone. Plus I moved my drone from the basement to the upper floor. Maybe there was interference downstairs?

Another good tip for you. Don't calibrate your compass in doors.. That's a good way of losing your air craft