Before your round, on the 'Round Setup' screen, you can edit 'Distance Units' by tapping on it. A message will pop up and there you can select 'Yards' or 'Meters'.
Note: The default Distance Unit is based on your location, as defined in your Hole19 account.
To change 'Distance Units' during a round: tap the ‘More icon’ (3-dots) > ‘Settings’ > switch the toggle off or on as you want to play in meters or yards. Tap ‘Save’ when you are done.