Yasmin here from Shopify Support :)
It's not possible to change the date of an order but you can easily create new orders instead. Go to Orders and Drafts in the admin and you will be able to make new copies of the old orders.
When you're done you can select "Mark as Paid" if the customers paid already, or e-mail them an invoice to have them pay now. Here's a detailed guide on using drafts: https://help.shopify.com/manual/orders/create-orders
Hope this helps!