Can I update my iPhone 6s to iOS 15

An iPhone 6s can be updated to iOS 15.8.1. An iPhone 6 cannot.

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Update your iPhone or iPad - Apple Support

If your iPhone or iPad won't update - Apple Support

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To update to the most recent, compatible version of iOS, 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. 

Settings > General > Software Update

Does an update appear?

If not, what message do you see?