To refund or cancel a deal, first find the deal and goto the deal overview screen.
Then click on the cog next to the deals status:
and then on either 'refund' or 'cancel':
Refunds - You will be offered the refund option if the deal has been completed and / or has payments marked as paid. Note that refunded deals will still be sent to any accountancy platforms linked with the system, to prevent this you will need to mark any payments as unpaid and then use the cancel option instead.
Cancellations - You will be offered the cancel option if the deal is incomplete and has not payments marked as paid.