This one is easy!
Go to Appearance >> Widgets and add a Text widget into Home Testimonial 1, Home Testimonial 2, or Home Testimonial 3. Add your text and save!
* If you want all of the boxes to be the same height, go to Appearance >> Editor, select the style.css file, and find the line:
/* Testimonials Height */
Change the value in min-height line to the minimum height of your boxes (may take some trial and error). Then, remove the /* and */ around it so the line is just:
(The demo uses the call-to-action button style for the “read more” links in this area.)