Step 1.
Login to your Customer Dashboard, Click on Billing

Step 2.
Click on "Add New" button and select a mode of payment from the drop down box.

Step 3.
Choose a Payment option from Card, PayPal or Bitcoin.
Bitcoin does not support auto renewal, So everything your VPN expires, you need to order new VPN.

PayPal and Credit Card supports auto-renewal option, You can toggle Auto-renewal ON/OFF from Subscription page.