@Mark535 (see corrected link)
When you cancel as host the penalties are high - having the days blocked is only one of them.
You thus can't clear them.
This is how to contact Airbnb helpdesk to explain that you did it to help the guest and have it upturned:
https://community.withairbnb.com/t5/Help/Contact-Airbnb-A-Community-Help-Guide-UPDATED/m-p/413245
In future, if your guest can't make a change, then easy - YOU can make it for them to accept:
go to the reservation, click CHANGE/CANCEL, and choose CHANGE (not cancel!), and you can change no. of guests, dates, price (automatic calculation is overwritable), Airbnb adds their charges accordingly.