You will have to wait until their visit is completed (possibly as long as 24 hours after check-out), then
1. Log into your administrative panel
2. Mouse over "messages" in the top menu and click "view inbox"
3. Find the guest and click on their last message to open up their trip details and messages.
4. If you have waited sufficiently after check-out you will see a box at the top of the page just above the last message that reads, "[Guest's] trip has finished. If you need to offer or request money for an issue from the trip, you can use our Resolution Center." Underneath that is a box that is a link to the resoltion center (Resolve an Issue).
5. Click the link.
6. On the resolution screen make sure the right guest is showing, then select a reason, in this case, "Offer partial Reimbursement."
7. Click "Continue"
8. On the next screen enter the amount you desire to refund and you can also add a note of explantation to the guest.
Airbnb will then send the refund offer and the guest can accept, decline, or request a different amount.
