Change taxonomy slug in database

You could just run DELETE FROM wp_terms WHERE slug LIKE "museum-%" on you MySQL server, but that would probably leave some orphan rows in wp_term_taxonomy and potentially in wp_term_relationships if you have used the terms. While being a bit more complicated I would recommend deleting them using the WordPress API. Something like this might work (from memory, so I’m not 100 percent on the usage of the term_ prefix in $term->term_slug and $term->term_id):

$terms = get_terms(array('your_term'), array('name__like' => 'Museum'));
foreach ($terms as $term) {
    if (term->term_slug != 'Museum') {
        wp_delete_term($term->term_id, 'your_term');
    }
}

It’s apparently not possible to search for terms by slug, so be carful not to delete terms like “Museums” or “Museum of Natural History” which will happen if you run the above code.