Your due date can be found near the top right-hand corner of the invoice, as well as in the footer just above the payment options:
If you have set up credit card or bank direct debit, confirmation of this will be found in the footer of the invoice and your payment will be deducted on the due date:

If no direct debit is in place, payment details and the due date can be found at the footer of each page of your invoice:

If you'd like to set up direct debit, go to the main menu in the Hub and click on Account, then Billing, and on the right, you will see box for Payment Methods:

Click on the button for New payment method and follow the prompts to enter your credit card or bank details.