set_post_type( int $post_id, string $post_type = 'post' )

Source

File: wp-includes/post.php

function set_post_type( $post_id = 0, $post_type = 'post' ) {
	global $wpdb;

	$post_type = sanitize_post_field( 'post_type', $post_type, $post_id, 'db' );
	$return    = $wpdb->update( $wpdb->posts, array( 'post_type' => $post_type ), array( 'ID' => $post_id ) );

	clean_post_cache( $post_id );

	return $return;
}