Wordpress menu….testimonials….Add New Testimonial
Usually somewhere like here:
/wp-admin/edit.php?post_type=testimonials
Add Title: name of person giving the testimonial
Content: would be the testimonial
Featured image: photo of person or a genetic icon
And save!
The ones you already see are the ones that come with the theme. Once you have a few testimonials of your own, you can safely delete theme.