Update my iPhone
How do I update iPhone 6s 13.7 to 14.0
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iOS 14 is no longer available to update to for any device. If you really have an iPhone 6s, then it can update directly to the current iOS15.6.1
If you have an iPhone 6 (no s), then it cannot update beyond iOS 12.5.6.
Either way you can go to Settings->General->Software Update and see what update if any it offers for your device.
Link-> Update your iPhone, iPad, or iPod touch - Apple Support