The easiest way to renew is by email.  You will receive notice when the ad expires, and if you use a fiarly recent email program you will see a button right in the email that will allow you to renew.  Just click the button!.

You can only renew non-paid ads this way.  If it is a paid ad, or you cannot see the button in the email, simply go to your account page at http://epage.com/js/account, and hit the "renew" link next to the ad.