how do you get a barn sour horse away from a barn?

Slowly introduce the horse to new places as you progressively work towards your goal of the horse not being barn sour. You do this by making it fun to be away from the barn or stable. See if the horse does better if he / she has a companion to go with them, maybe take a special treat along with you and offer it to your horse when it is being good and not trying to get back to the barn. Or simply change your routine.