If the option for Auto Renew is turned on, your domain will renew, as long as your account is funded. To turn on Auto Renew, log in to your account, choose domain management from the menu options, and find the domain name you wish to enable auto renew on. Click on the 'm' button next to that domain. On the next page, check the box for Auto Renew Enabled.
To fund your account, log in to your account and click on refill account on the right menu. Enter the amount you wish to refill, between $10.00 - $100.00. Proceed to create an invoice. Once you pay the invoice, your account will be refilled.
In order for the domain to successfully auto renew at the end of the year, you must have auto renew turned on PRIOR to 30 days before the expiration date.