You can extend the service time via:

If you don't have an outstanding invoice but want to pay ahead of time please contact our support team who will generate a invoice for you. 

Within the new web page you will find two sections "Outstanding Invoices" & "Paid Invoices" you will only want the Outstanding and select "Pay Invoice

You will then be directed to a new page where you can review the overall cost of the service and enter your "Billing Details"

Locate at the bottom there is the option to setup the auto renewal feature or single time payment and to "Add New Card" or use a existing one for the payment. 

The last section will be a review of the invoice and will allow you to process the final step, if you are paying through Paypal you will like be prompted to login first, if its a card transaction this will proceed like normal.