You'll want to factory reset it. You don't state what model it is, but there's usually a reset button somewhere on the back, bottom, or edge if it's a stick. Sometimes it's behind a pinhole and you'll need a paperclip or something similar to press the button. With the power connected press and hold the button in for no less than 30 seconds. That should do it.
https://support.roku.com/article/208757008-How-to-factory-reset-before-returning-selling-or-gifting
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