To change the text of the button, you will need to apply custom CSS or JS code to your Careers page. In your Careers page CMS click on Edit settings on the left hand side of the screen to expand menu. In this menu select Custom CSS or JS option.
Add the following code to Custom style / CSS field:
content: 'Your custom text';
Replace "Your custom text" part with the text you want to appear inside of the button and save. "Apply for this job" buttons on all jobs will now have this text. Note that you can't set different texts for different jobs this way.