IMPORTANT NOTE: Please understand that the reason you're marking the order as 'Paid' is because it's a duplicate order that your customer has already paid for. You don't want to charged them again, and the amount has already been collected in Shopify.
If you cancelled an order that was never fulfilled:
Head over to the 'Orders' page.
Expand the order in question by clicking on it.
Click the 'Undo Cancellation' button.
If the order was cancelled during processing (fulfillment amount refunded):
Head over to your Shopify backend.
Find the order in question then click on it.
Click the 'More Actions' dropdown.
Click the 'Duplicate' option.
Add the correct customer.
Click the 'Collect Payment' button.
Click the 'Mark as Paid' button.
Click the 'Create Order' button.
Head back to Zendrop and refresh to see the duplicated order.