Hi, Patrick!
I had a similar case in December with the cancellation of a March reservation. Guest cited concerns related to COVID. I also was not aware that our flexible cancellation policy does not apply to longer reservations (lesson learned!). I had already told the guest that they would get a full refund upon cancellation. I contact support via the app on my computer and was able to get help to resolve the issue.
To submit a ticket without calling, go to the Help Center in the Airbnb application on your computer, then scroll all the way to the bottom of the screen. There should be a button on the bottom right of your screen called "contact us". You can submit a help ticket there without having to call them.
Good luck & happy hosting!
All the best,
Jennifer