Change button text from "I'm Attending" to "Sign Up"

My client's site visitors cannot understand that clicking "I'm Attending" means "Sign Up". How can I easily change the button text, and will I have to change it every time there's an update to the plugin afterward?

It would also be helpful if you could add some text before and after the events listings. My client would like to put her phone number below the listings (for questions), and some instructions above the listings.