You can create and manage your payment methods from the menu Settings > Payment methods:
You will find the list on the current payment methods:
You can create a new payment method by clicking on the 'Create a Payment method' button:
Enter the name of the payment method and click on 'Available in the POS' if you wish to have it available in the POS app. You can find more information about the POS on this page.
You can edit a payment method by clicking on the Edit button:
You can disable / Enable a payment method by clicking on the 'Disable / Enable' button:
Please note that you can also create your Payment methods directly when adding a payment in the invoice. Simply start typing the text and press enter or click on the underlined text: