If the host has not accepted the invitation yet, you can delete the host and invite a new one. To do so, go into the event and go to the DETAILS page. Click on the trash can once you are hovering over the host box.
When you hit the trash can, you will see this pop up:
Click REMOVE if you want to delete the host from the event.
Then you will be able to add the new host!
If the host has already accepted the invitation, you have to email firstname.lastname@example.org and they will take care of it right way.