iPhone 6s update

To update to the most recent, compatible version of iOS 15.7.3, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

adu ~ iPhone 6s cannot be updated beyond iOS 15:

iOS 16 and iPadOS are compatible with these devices

...Found by searching Apple's Knowledge Base.

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