Device registration process for me

Perform a Factory Reset to put it into new_out_of_the_box condition.

Factory reset is accessed via Settings > System > Advanced system settings > Factory reset.

If you don’t have sufficient control to navigate the Roku's menus, find the reset button on the Roku itself. On some units it may be a pin hole you must use an unbent paperclip or something similar to access.  Press and hold the reset for 10 seconds to start the 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.

As part of the setup process you can link it to your existing Roku account if you have one, or create a new one if not.