@Monica1700 the sad fact is that these sorts of guests target new hosts. When you first start out its always better to limit your stays to shorter duration for two reasons: it helps you build up your reviews and it makes it easier if you do run across a poor guest. So you might want to rethink that. You should also be mindful of the tenancy laws in your area. In many states a guest becomes a tenant after 28 to 30 days. That is a whole 'nuther issue.
I would send the change request to the guest with the following message:
"As we discussed, here is the cancellation option for the remainder of your stay. I am sorry that my space was not a fit for you. I have let Airbnb know that you will be leaving early. Also, I am happy to resolve this as you requested, but bear in mind that your mention of leaving a poor review unless I comply with your wishes is a violation of the Airbnb Terms of Use. I wish you well and hope you find a great place that works for your needs."