You access this item in the testimonials manager.
You can get there by logging into the /backend, selecting content from the 3 line menu, then click on "tools" on the left-hand side, select "Testimonials". You can also click the big blue '+ Add New' button and choose 'Testimonial'.
Use the WYSIWYG editor to add testimonials and rave reviews you've received from your clients. This featured is designed to show a new testimonial at the top of the page each time it is loaded.