Programatically change post author

Given a user id, $user_id, and post id, $post_id, how can I programatically update a wordpress post author?

Note: these posts are already created and the author cannot be set upon creation. Another process is creating the post and defaulting to the admin as the author. I don’t have access to creating the post but have access after it is created.

It shouldn’t be any problem. Try this:

$arg = array(
    'ID' => $post_id,
    'post_author' => $user_id,
);
wp_update_post( $arg );