Compass setting in the missionplanner

when i choose the compass with external
it is always changed to the setting of manual.
can you tell me how to change it?
thank you!

What flight controller and GPS are you using?

APM and Ublox M8N GPS

What exactly is changing to “manual”?

There are four settings in missionplanner, I change the setting to onboard setting.
after another setting, it is automatically changed to manual setting.

Depending on the APM board you have, the switch between internal and external compass is not software controlled but rather a physical setting. Depending on the version you can select internal/external compass via a jumper connector, others you need to solder a jumper.

There are a gazillion settings in Mission Planner. Post a screen shot of what you are talking about.