WordPress - Update post programmatically when a post is saved

wp-update_post-on-save_post.php

<?php 

function update_on_post_saved( $post_id ) {

    if ( wp_is_post_revision( $post_id ) ) return;
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return;
    // if ('NNN' !== $_POST['post_type']) return; return if post type is not NNN

    // unhook this function so it doesn't loop infinitely
    remove_action('save_post', 'update_on_post_saved');

    // Update post
    $my_post = [
      'ID'           => $post_id,
      'post_content' => 'Put content here',
    ];

    // Update the post into the database
    wp_update_post( $my_post );

    // re-hook this function
    add_action('save_post', 'update_on_post_saved');
}
add_action( 'save_post', 'update_on_post_saved' );

Glad it helped!