To change your domain/URL for an event site you follow these steps:
Click ‘Settings’ > ’Setup’
Scroll down to Domains and click ‘Update domains’
Click ‘Add domain’
Enter the Path you want and click ‘Add domain’.
The new one is automatically set to Primary. You can either leave the old one as redirect or you can delete it by clicking on it and then ‘Delete’.
If you want a custom domain, read this article!