There's nothing you have to do, it will change back to the normal plan on its own. I am on the 6-month promo with double data as well but there's nothing that has to be done. Once your 6-months are up it will switch on its own.
Hello Angel Fish, Knight Rider is correct, your package will transition on its own. There is nothing that you have to do directly to make this happen. There is a chance that you do receive an email when this does happen on your account.