Help getting started with Roku devices, including setup, connecting your device to your TV, linking a Roku device to your account at my.roku.com/link, adding channels, subscribing to services, and more.
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When activating your Roku device, you should follow the Quick Start Guide and complete the on-screen instructions to connect your Roku device to the internet. Once this is complete the Roku device will display a link code on screen to link your device to your Roku account.
If the previous owner didn't do a factory reset, you should do so. I believe that will also provide what you're looking for. Go into the Settings menu and look under System/Advanced System Settings for Factory Reset. This removes all installed channels, any user information, and gives you the same startup process that a brand new device would have.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.