Hi there, Brian! Thanks for getting in touch.
Good question. We recommend using template overrides when customizing calendar templates. By placing copies of the templates in your theme (or child theme) folder, you are ensuring that your work will not be overwritten when the plugin is updated.
The detailed steps for template overrides are outlined in our Themer’s Guide. I’d suggest checking this out to see all the available options you have for customizing the plugin without touching core files.
Cheers!
Geoff