Software

Assuming you have a compatible device, 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. 

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Check better: are you sure your iPhone is a 6s model? (not a 6, which cannot support iOS versions beyond 12.5.6)

What iOS version is currently running on your iPhone? check: Settings>General>About>Software Version.

Regards

Giulio

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CLIESFRAFOR ~ Apple offers many newer iPhone models which can run iOS 15:

iOS 15 and iPadOS 15 are compatible with these devices

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

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As posted by Giulio above, the iPhone 6/6 Plus can be updated to iOS 12.5.6, no further.