In general, to renew a domain name once it has past its renewable period, you simply need to pay for its renewal invoice. To pay for an invoice, you need to login to your client area and click on Billing > My Invoices.
If no invoice is due for the domain in question, please open a ticket for the Billing/Accounts Dept., our Billing team will be able to assist you further. Please note that certain domains (such as .eu domains) come with an additional fee to renew if they have not been paid by their due date. If you have any queries on this, a member of our Billing team will be happy to assist.