You will have to first create a testimonial via Admin Dashboard > Testimonial > Add New. Enter the title, the content and upload the featured image.
After you have created the testimonial, go to Appearance > Customize > About Page Settings > Testimonial Section.
- Enter the title of the section in the Section title field
- Enter the description in the Section Description field
- Click on Add new Testimonial to add a testimonial
- Select the testimonial in the Choose Testimonial drop down option
- Click on Publish.