Refunds are processed by host administrators. If you are not the admin, contact your fiscal host (or Open Collective support if your host is managed by Open Collective).
To refund a donation as a host admin:
Go to the Collective page and find the budget section
Click "all transactions"
Find the one you want to refund on the list
Click 'View details'
Then you'll see the 'refund' button
Click it and confirm