iPhone 6s won't update
Why won't my iPhone 6s Plus update to iOS 13
Question marked as
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.