Go into your Theme HTML/CSS by following the directions here.
Find your course_action_buttons file. It is in the Sections folder.
You'll want to modify the code on line 36 and remove the default text with your own custom text. Make sure to keep your text in quotations for the code to run properly. For example, in the screenshot below, the course button will now show "Preview the course for free" if the course price is set to free:
Don't forget to save the file, and to make a note that you've made this change in case you want to go back or update your Themes.
Preview your site to make sure it works! If you have a question about this or another Theme customization, write us a note here: http://help.thinkific.com/support/tickets/new