Be sure that you are in the "Songwriting" mode of the app.
To enter the chord, edit overlay, double-tap on a chord, in a bar, or in the wheel. Or tap on the "Modal Interchange" icon.
This is the "edit chord" overlay:
Tap the ☑️ icon to save changes.
Tap the ❌ icon to discard changes. This will revert the song to the state it had before you have opened this overlay.