change-post-type-slug.php

<?php
add_filter( 'registered_post_type', 'my_portfolio_cpt', 20, 2 );
function my_portfolio_cpt( $post_type, $args ) {
	if ( 'portfolio-item' == $post_type ) {
		global $wp_post_types;
		$args->rewrite->slug = __( 'resources', 'my_theme_textdomain' );
		$wp_post_types[ $post_type ] = $args;
	}
}