You have to factory reset the stick to link it to your account. You can do that from Settings->System->Advanced system settings->Factory reset. I don't think your father has to do anything, but it wouldn't hurt for him to go to his account at https://my.roku.com and unlink the stick from his account.
If that was your father's only Roku device he should close his Roku account and ensure he doesn't have any active subscriptions ( https://my.roku.com/account/subscriptions ).
How to close your Roku® account | Official Roku Support
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