To issue a refund you need to open the order.

Bookings => Open the Order 

There are three ways of issuing a refund

Cancel the Booking for a full refund

By cancelling the Booking you automatically refund the user.

Cancel the order and manually refund

If you cancel the booking, the money stays in the order and you can process a partial refund 

When you cancel an order, the booking balance is still positive. Now you can rebook the customer on a different product and sort out the difference (pay or refund the balance)