iOS 14.0 connect

If it is an iPhone 6S Plus you can update the iPhone to iOS 15, see this Apple article:

iPhone models compatible with iOS 15

iPhone models compatible with iOS 15.4 – Apple Support (UK)

Go to: Settings -> General -> Software Update.

See this Apple article: If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-us/HT201435

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

You need good Wi-Fi connection. 

If they are On, turn Off Hotspot & VPN.

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.