As part of making a multi-annual order for a domain (instead of single-year), I needed to create manually a domain renewal order. Here are the steps I used.

First, log in onto your control panel (if you're not already logged in).

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Then, form the list of owned domains, select the domain you want to create the new order for.

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Click on the "Renew domain" button.

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Select the period of time you want to renew the domain for (3 years in my case).

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Select the preferred payment method and "Place order".

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You can pay the order on the spot or you can use the e-mail you receive to pay it later.

NB: Just remember that you're on your own if you let your domain lapse.