How do I change the ‘Next testimonial…’ text

In your theme’s functions.php file, add similar code as follows.

add_filter( 'testimonials_widget_premimum_next_text', function() { return 'Next →'; } );

or

function my_content_next() { return 'Next…'; } 
add_filter( 'testimonials_widget_premimum_next_text', 'my_content_next' );