If your domain is registered with us and it is set to auto-renew, it will renew automatically about a week before expiration with no action needed on your part, provided there are sufficient funds in your account. (You'll be warned repeatedly and often if the necessary funds are not available.)
If your domain is not registered through us, you will need to contact your registrar to renew it.
Remember, any renewals add on to the existing registration. So if it is May 2010 and your domain expires in August 2010 and you purchase a one year renewal, your new expiration date will be in August 2011, not May 2011. There is no penalty for renewing early!