The option 'refund guest' doesn't appear under guest's name, though he has cancelled before check in date. If I go to resolution page and go to that reservation, the drop down box for 'reason to start resolution' doesn't list give guest money back! So what now? And what is the general contact number for airbnb, and why is it so hard to find on the website!!! Grrr.
You don't need to do anything if the guest cancelled.
If a future guest cancels, Airbnb handles the refund of the guest in compliance with your cancellation policy. You do not refund them. Seing that you have a flexible cancellation policy, the guest should receive a full refund minus Airbnb's fee.
I would refer you to these two Airbnb help documents: