How to 'deauthorize' my MacBook Air, OS High Sierra

I am sending my MacBook Air in to Apple to get a payment toward a new MacBook Pro. The instructions on how to prepare the old unit for shipping begins with 'Signing out of iTunes' and 'deactivating' iTunes by going the header in iTunes and selecting 'Account' > 'Authorizations' > 'Deauthorize this Computer.'

My copy of iTunes doesn't have this heading. Where is this process located?

Thanks.