iPhone 6s is not working in version 15.6

For sure check your iPhone model & iOS version -> Go to: Settings -> General -> About.

If your iPhone is an iPhone 6S Plus you can update the iPhone to iOS 15.7.3, see -> iPhone models compatible with iOS 15.5 -> iPhone models compatible with iOS 15.5 – Apple Support (UK)

You need about 6 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

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

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