In most instances, AvPlan EFB will submit the callsign provided by the selected aircraft profile. The ability to override that callsign for flight plan submission purposes is designed for use by Military or Emergency Services that always use the same callsign, regardless of the particular airframe being flown.
Tap Planning > Submit ICAO Flight Plan via NAIPS.
If you have inadvertently changed or overridden the default callsign, you'll notice the word (Overridden) next to the Callsign text:
To remove the overriding callsign, tap the callsign text:
Use the backspace key to remove all characters. With no more characters (just a flashing cursor) tap the Return key.
The 'Change Default Callsign' popup will appear. Ensure you tap Yes to confirm the change.
The 'Overridden' message should disappear, and the aircraft's correct callsign should now be displayed in the field.