iPhone 6s won't update
Why won't my iPhone 6s Plus update to iOS 13
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How certain are you that you have an iPhone 6s? Could it be that you have an iPhone 6 instead and not the "s" version?
I ask because the iPhone 6 goes up to iOS 12.4.x and iPhone 6s and newer models go up to iOS 13.4.x.
To validate the model of your iPhone, open Settings ➔ General ➔ About ➔ Model Name and note what it states there.
Axel F.