How to change custom post type slug

You can use register_post_type_args to modify slug of any custom post type.

You need to change portfolio slug to work? Then code should be like this.

add_filter( 'register_post_type_args', 'change_slug_portfolio_to_work', 10, 2 );

function change_slug_portfolio_to_work( $args, $post_type ) {

    if ( $post_type == 'portfolio') {
        $args['rewrite']['slug'] = 'work';
    }

    return $args;
}