It's far too easy to create a Roku account in the wrong region. The only way to rectify it is to factory reset the Roku, create a new account, and link the Roku to the new account. If you have any subscriptions through Roku you'll want to cancel them and then close the current account. I wish I could say exactly how to prevent it from happening when you create the new account. You'll want to clear your cookies before you create the account at https://my.roku.com/signup .
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