Sign in here > https://account.sonyentertainmentnetwork.com/liquid/login.action
At the top, click on "Account"
Scroll down and click on "Media and Devices"
Click on "Deactivate All" and confirm if you are sure.
You can only do this once every six months.
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Just an FYI, game sharing breaches Terms of Service, maybe give some thought to that before activating someone else's console as your primary.