We are unable to download a book for purchase from the iBook store because it says you are not authorized to use the iBooks United States site and must use the Canadian site. How do we do this?

We have always been able to purchase books from iBooks no problem but all of a sudden it says you cannot purchase this book as you aren't authorized to use the US store and must connect to the Canadian store. We cannot figure out how to do this. Help!

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