I finally tuned my quad with the apm2.6 board with version 3.1.2
But when I flew in loiter mode, my quad tent to do the toilet bowl dance. This was because the magnetic north changes because of the magnetic interference caused by all the electronics onboard.
The solution to this is to do compassmot stated in the 3dr website. What this does is tells the compass the correct north by not registering the magnetic interference.
after doing compassmot.....
now one more problem has come up.
after the compassmot my interference was 5%, but now my motors are not spinning fast enough to take off even on highest throttle.

anyone to help me ?

this is my problem
https://www.youtube.com/watch?v=SbleSREn2Ew&list=WLdWjlIyXeFXxy_vDRrNheOg&index=42

There is no compass onbaord the APM2.6

are you using an external compass?

Yes external compass

Have you tried resetting APM and reflashing firmware?

Yes, I tried reflashing and reset.
My mission planner is unable to set flight modes.

use arduino to upload the firmware

I am selling this stupid stuff