How do I change the more content ellipsis

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

add_filter( 'testimonials_widget_content_more', function() { return ' Continue reading →'; } );

or

function my_content_more() {
    return ' Continue reading →';
}

add_filter( 'testimonials_widget_content_more', 'my_content_more' );