First, enter the chord edit overlay.
You can set a different bass note for a particular chord. If you do so, you will create a so-called "Slash-chord". Slash chords can sound awesome. You can even create odd chords by selecting a bass note which is not within the chord.
To access the bass note editor, enter the chord edit mode ant tap on "Bass note":
You can now select a different tone as a bass note. Initially, only tones that are part of the chord are displayed:
If you want to set a bass note that is not part of the chord, you can open the 🔒 "lock" to have every tone available.
Tap this icon to remove the bass note from a chord: