In that theme all the testimonial data comes from _data/testimonials.yml file and you need to change the content from there.
For example :
- name: Mr. Joe image: /assets/img/review/1.jpg review: Themeix Live Chat option was really helpful for me. y. designation: Generel Customer rating: 4
Here following the details about the keys used there.
name : It will appear as testimonial author name.
image : It will be the profile picture of the testimonial author or person or the company.
review: That is the main content of testimonial
designation: It is basically the subtitle of a testimonial author
rating: it will display the star ratings in the frontend.