Yes, you can change an order that has already been submitted unless it has the status of Being Fulfilled. Click the Change order button from your order view and edit the products and shipping options from there.
Note that changing a product within an order (e.g. altering a product's variant, size, design file, or mockup) that has the Waiting for Fulfillment status, or setting its status to On Hold, will reset the order deadline. This may result in longer fulfillment time. However, changing a shipping address or rate will not reset the order deadline.