Go to Income > Sales Orders, find the open order and click "Convert to invoice" in its action menu.

You will be directed to the invoice editor screen, where you can see the data in the original order are automatically brought to make a new invoice. 

Now specify the Due Date in order to put it in the book and proceed to payments after, and Currency Rate if issuing the invoice in another currency.

Of course, you can add some changes to your new invoice if the customer wants to order differently.

On more details to issue new invoices, read this article.