Update the Image Uploaded in frontend or Regenerate Thumbnails

$fullsizepath = get_attached_file( $attach_id );
$metadata = wp_generate_attachment_metadata( $attach_id, $fullsizepath );
wp_update_attachment_metadata( $attach_id,  $metadata );

if ( is_wp_error( $metadata ) )
echo 'Images not upoaded Correctly Please try Again.';
if ( empty( $metadata ) )
echo 'Unknown failure reason.';