How to convert custom fields to content

Question

You can do this by using something similar

<?php
//Post we need to update
$id = 37;

 $custom_field = get_post_meta( $id, 'custom_field_key', true );
// Update post 37
  $mg_post = array(
      'ID'           => $id,
      'post_content' => $custom_field,
  );

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

//Delete the custom field
delete_post_meta( $id, 'custom_field_key' );
?>

Hope you got the idea. Let me know what have you tried.