The lending library determines whether or not your item can be renewed. The sticker on the front of your ILL item will indicate whether renewals are allowed. If allowed, ILL items can only be renewed once.
To renew your ILL item, log-in to your ILLiad account and click on "Checked out items." Next, click on the item you want to renew. The request will open, and you will see a red link at the top of the page that says "Renew Request." Once you click on this link, you should be given a tentative new due date.
Once the lending library confirms the renewal, you will receive an e-mail with the new due date. Remember, it is always possible that the lending library will deny the renewal (though this only happens on rare occasions).