Thank you for the feedback. Currently, the Register Now button cannot be edited on the Event Registration Form but it can be changed on the invitation and landing page. Here's how you can do that: edit the block where the link/ button is saved > highlight the words "Register Now" > click the link icon on the toolbar > change from "Register Now" to "Buy Now" > click Save/ Update.
Please continue voting for this feature so we can make the registration form button editable like the other pages too.