To switch an existing Roku setup to a different account you have to do a Factory reset to clear the existing account information from that unit.
Factory reset is accessed via Settings > System > Advanced system settings > Factory reset. This will clear out the unit's memory and you'll then have to set it up again as though it were a new unit right out of the box. During this process you can set up a new account and link to it, or you can link to an existing account you have set up ahead of time.
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