The only way to change the Roku account a device is linked to is to factory reset it, so you'll have to do that with one of the devices. There's no way to merge two accounts. You'll want to cancel any subscriptions you have through Roku for the account of the one you factory reset and maybe close the account if you don't plan on using it again.
How to factory reset your Roku® streaming device | Official Roku Support
How to close your Roku® account | Official Roku Support
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