By default the system allocates “Due on Receipt” as the payment term for all the invoices. You can change the payment term by getting into the invoice creation page. Please refer the image below:

If you wish to set default payment terms to your customers, then you can associate specific terms for each customers. Following are the steps to configure the payment term at the customer level:

• Click the "New Customer" button from the "Customers" page.
• In the "Payment Terms" drop down, click and select the preferred term.
• Click the Save button.