You have full control to edit and delete any testimonial via the Merchant Center.  Please be advised that as a Basic free user, you cannot delete a testimonial.

To edit or delete a testimonial take the following steps:

Login to the Merchant CenterHomePageLogin_2016-05-22_1058
To edit a testimonial click on the pencil icon.  You will be taken to the update testimonial screen

Make changes to the testimonial, then click “Save” to save the changes

EditTestimonial
UpdateTestimonial_2016-05-23_1059
To delete a testimonial click on the garbage can icon.Delete_Testimonial_2016-05-23_1107