In iOS 11, you don’t have to. Just let the observer go out of scope. There is no penalty any longer for letting an observer die before the observed or for letting the observed die before the observer, so you have no actual work to do.
On the other hand, if you really want to unregister the observer, remove it from whatever is retaining it, or tell it to invalidate. (Something must be retaining it, because if you don’t persist the observer, it will die and your observer function will never be called.)
(You say “if I store an observer like this”, but the way you are storing it, with let, is a somewhat silly way to store the observer. It would be better to put it in a Set from which you can remove it later, or at least store it in a Optional var that you can later set to nil.)