How to copy data from a custom field to the post_content?

First get data from wp_postmeta table

$description = get_post_meta( $post->ID, 'description', true );

Try following code to update content

$my_post = array(
  'ID'           => $post->ID,
  'post_content' => $description,
);

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