Can I update a 6s to 15

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You can update an iPhone 6s to IOS 15.

See the articles below.

Update your iOS device.  -

Get help with over-the-air iOS updates  -

Update – Can’t or can’t restore   -

Have you tried using iTunes on a computer or Finder if running Catalina or later to do the update?

Update software on iOS devices in iTunes on Mac. -

Phone, iPad, or iPod touch - Update with Finder.  *

iPhone, iPad, or iPod touch - Update with iTunes.  *

If you see an error when you update or restore your iPhone, iPad, or iPod    *

Updating iOS: common issues and solutions.  -

ASC YouTube for  how to update your iPhone, iPad, or iPod touch to the latest version of iOS or iPadOS —wirelessly or using your computer.