In case (for example) you have a song in C but the default key signature shows D, you need to set the default key of the song correctly.
Go to the song, open the Editor, tap on 'i' info button, then the Key and select C and tap on "Set".
Only the Default key signature of the song will be changed – without changing any of the chords; the default key you have set for the song should then correspond to the chords as they show in the song editor.