User Post Type – change post type slug from “upt_user” to just “user”

<?php

/*
    Add the following to your (child) theme's functions.php

    You may need to flush WP's rewrite cache afterwards. To do so,
    browse to Settings > Permalinks and simply hit "Save Changes".
*/

add_filter( 'upt_post_type_args', function( $args ) {
    $args['rewrite']['slug'] = 'user';
    return $args;
});