How do I update my iPhone 6s

Go to Setting > General > Tap Software Update .

To avoid unnecessary error , plug your device into a power source and connect to Wifi.

Make sure iPhone have Enough Storage for update at least 5 GB.

Here --> Update your iPhone, iPad or iPod touch

Marking your own post as Solved may prevent others from trying to help because they think you have fixed your problem.

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.