Hello Cas,
Thanks for reaching out! I’m happy to help you with this one.
When you are editing an event, then in the top right corner of the page you can see a “Screen Options” button. If you click on that, then a frame opens.
In that frame make sure that the checkbox next to “Author” is checked. This will make a meta box appear somewhere on the event editing page (likely at the bottom) where you can choose a new Author for the event.
Hit Update and you are done.
Here’s a screenshot to guide you.
Hope this helps, let me know.
Cheers,
Andras