On the POS screen at the top is a red button 'History / Refunds' as shown here.
When you tap on it all of the transactions performed over the last 24 hours on that device will be displayed on the left of the screen as shown below.
If you then tap on the transaction on the left side that you want to reprint the receipt for all of the products of that transaction will be listed in the cart on the right. Just tap on the 'Reprint' button at the bottom of the cart to reprint the receipt. Alternatively you can refund the transaction.
If for some reason you need to reprint or refund a receipt for a transaction made over 24 hours previously then you'll need to locate the transaction in the backoffice. Once you've found it click on the 'Details' link to the right of it.
This will open an expanded, detailed view of the transaction with the transaction number at the top left.
On the POS screen tap on the 'History / Refund' button at the top of the screen and type the number into the search box as shown below.
When the transaction is found it will appear at the top of the list of sales on the left with the products shown in the cart. Just tap on the 'Reprint' button and a new receipt will be printed or tap on the 'Refund' button to refund the transaction.