When inputting a chart, you use the minus (hyphen) button on the editor keyboard (Editor panel on Mac, or hyphen key) for the minor symbol. Then choosing this global setting (it applies to all songs), it will show minor as either the hyphen or "m" according to which you select.
For example, input F-6 in the editor. It will show as either F-6 or Fm6.
iOS, Mac, Android: Tap/click the Chord Chart Settings button (to the left of the edit button on the top of the screen) (on Android it is labelled "A")
Then tap/select the Minor Symbol button to change it to 'm' or '–'.
Mac 2023.7 or below: iReal Pro >Settings (Preferences) > General - click Symbol for minor chords for 'm' or '–'.