If the Host has not already accepted the invitation into your event you can delete the Host by logging into your event and going to the Details page. Hover over the Host and click on the trash can to delete them.
When you click on the trash can, you will see this popup:
Click Remove if you want to delete the Host from the event. Now you can add a new Host!
If the host has already accepted the invitation you will need to email firstname.lastname@example.org and they can remove the Host from the event on your behalf.