We’re straying off topic here, but that’s not the case — iOS will download updates automatically, but it will never install them unless you give permission. It might even nag you incessantly, but it won’t install unless you agree.
That said, if you’re having bad battery life on an iPhone 6s, you should get a new battery. They’re just $29 from Apple. See “Apple Apologizes for iPhone Battery Issue, Drops Replacement Prices to $29,” 3 January 2018.