To overwrite your post type slug, use the following function and refresh your permalink structure.

overwrite-post-type-slug.php

<?php
/**
 * Overwrite custom post type slug
 *
 * @param array $args
 * @param string $post_type
 * @return array $args
 *
 * @link https://wordpress.stackexchange.com/questions/247706/how-to-change-custom-post-type-slug
 */
function overwrite_post_type_slug( $args, $post_type ) {

	if ( $post_type === 'release') { // replace "release" by the post type slug you want to change
		
		$args['rewrite']['slug'] = 'spektakle'; // replace "spektakle" by your desired slug
	}

	return $args;
}
add_filter( 'register_post_type_args', 'overwrite_post_type_slug', 10, 2 );