You can add Testimonials to your book if you go to this page:
...making sure to replace YOUR_BOOK with your book's unique web address.
You can also find the "Testimonials" page listed under "Community" in the Overview page for any Leanpub book:
Here's what you'll see when you first go to this page:
Click the "New Testimonial" button to go to the page where you create a new testimonial:
Next, add whatever information you like to the testimonial, and click the "Create Testimonial" button.
You will now see the testimonial listed on the main Testimonials page:
If you have more than one testimonial, you will be able to change the order in which they appear one the book's landing page. Just make sure to click "Save Positions" after you re-order them.
Here's an example of what a testimonial looks like (though they're actually smaller than this image probably appears in the article here):