Setting a default payment method means that the payment method will be pre-selected when creating a New order:
To set default payment method
Go to Settings > Admin > Payments & Finances > Payments
Select your preferred default payment method
Press Submit to save changes
When you go to place an order via the POS on the new order page, it should now look like the following depending if you have the payment popup enabled or not.
With the payment popup enabled:
With the payment dropdown menu enabled:
