Good evening Michael and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
I would love to help you with this topic.
Actually, as my colleague Cliff suggested, adapting and using the snippet found at the bottom of this page in your theme’s functions.php file should work: https://theeventscalendar.com/knowledgebase/change-the-wording-of-any-bit-of-text-or-string/
The other solution would be to use your modified rsvp.php file and putting it in your WordPress theme’s folder. You might want to read our Themer’s guide to get a sense of how that works.
This way, the file will not get updated when you upgrade the plugin.
Let me know if that helps.
Have a great day!
Geoff B.