How do I configure Next and Previous page indicators

For previous page links, in your theme’s functions.php file, add similar code as follows.

add_filter( 'testimonials_widget_previous_posts_link_text', function() { return 'Previous'; } );

or

function my_testimonials_widget_previous_posts_link_text() {
    return 'Previous';
}

add_filter( 'testimonials_widget_previous_posts_link_text', 'my_testimonials_widget_previous_posts_link_text' );

For next page links, use testimonials_widget_next_posts_link_text instead of testimonials_widget_previous_posts_link_text in above.