The setup process I am sure is available someplace on line if you google the manual for your autopilot.
That said, my guess along with the other advice you are getting is that if the fluxgate is reading that far off (I'm assuming your binnacle compass is reading good here), it is likely that is is close to something magnetic. If it worked well before, you may have moved a magnetic object close to the fluxgate sensor in the boat. Not that I've ever done that
Other possibilities, all of which have happened to me on different boats different years.
- I've had a builder place the fluxgate sensor too close to a deck wash pump, which has magnets in it of course.
- I've had a fluxgate fail. Some of them have a gimbal in them to keep the sensor level, and I've had that break internally to the fluxgate sensor.