It is done during activation. No need to notify Roku of anything. After you complete activation, you can navigate to https://www.roku.com/ and then login to your account and see your device activated details, etc.
You can use multiple Roku's in a house. Not limited to one device. If you are keeping as a spare, just unplug and put in a drawer. If you plan to give away to a stranger, then factory reset the old one and deactivate it under your account. (again, at https://www.roku.com/)
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102