How do I get a preferred update (iOS 10) on an iPhone 5S rather than the latest version?

In general? You don’t. Once Apple releases the latest version, they stop signing the older versions, which makes it impossible for you to downgrade to the older versions without significant effort and skill. This is done to ensure that the majority of the user base is running the most secure versions of the operating system, which makes iOS devices much harder to attack.

iOS 11 isn't that much slower on the iPhone 5S:

Speaking strictly in qualitative terms, using iOS 11 on an iPhone 5S doesn't feel bad. You notice the impact of background tasks more than you used to—updating apps or downloading a bunch of music in the background makes the whole phone feel sluggish. But taking pictures, sending e-mails, and scrolling through apps is still more than responsive enough to be usable, and I rarely find myself wanting to throw the 5S through the nearest wall like I did with the 4S a couple of years ago. Compare the load times for some of the apps above to the load times for the same apps on the iPhone 5 or iPhone 4Sunder iOS 10 or iOS 9. The 5S is still significantly faster, even with iOS 11.

So there’s not much of a good reason to avoid running iOS 11 on the iPhone 5S, because there are many security vulnerabilities that have been fixed in iOS 11 that are absent in iOS 10.

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