Training

 In order to change the author of the testimonial to someone else besides you (or the creator of the testimonial), when you are editting or creating a testimonial from the backend, remove all the text in the Author field, and then start typing the person's name. As you type, an autocomplete dropdown will appear, and you must select one of the contacts in the dropdown

If the dropdown doesn't have the person that you're looking for, most likely that means the person is not in your database, so you have to create a new contact for the person first, and then try the autocomplete field again.