I’ve been meaning to find a way to do that and your question motivated me to figure it out, so thanks!
Until I can build it into the plugin settings, add this to your theme’s functions.php
:
/*
* Change slug for Strong Testimonials
*/
function strong_change_slug() {
add_permastruct( 'wpm-testimonial', "review/%wpm-testimonial%", array( 'slug' => __( 'review', 'my_theme_text_domain' ) ) );
}
add_action( 'init', 'strong_change_slug' );
Then flush the rewrite rules by either deactivating & reactivating Strong or by going to Settings > Permalinks and just clicking Save.
In case you want to change the slug again, be sure to change both instances of “review”.