The only way to change the account a Roku device is linked to is to factory reset it. If you don't recognize the account (the email address displayed under Settings->System->About), then you should definitely factory reset it/them.
How to factory reset your Roku® streaming device | Official Roku Support
If it is your account, you can change or remove the PIN by logging in at https://my.roku.com using the email address displayed under Settings->System->About.
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