Why my phone (iphone6) is not updating? What's app , Facebook are requesting for updates

Your iPhone 6's hardware cannot support the newer version of iOS. If you need to run apps that require a newer version, you will need to get a newer iPhone model.

Ref: iOS 16 is compatible with these devices - Apple Support

Welcome, ritwik160, to Apple Support Communities!

An iPhone 6 should be able to be upgraded to iOS 12.5.5, or so, provided you don’t have a Beta profile installed on your iPhone.

Are you not being presented with the option of upgrading to iOS 12?

You can use a computer (PC using iTunes, or Mac using the Finder for Catalina and newer, or iTunes for older MacOS systems). That can get past many issues.

If What’s App and/or Facebook are requesting a version of iOS beyond 12 something, then your iPhone 6 is simply too old, for this software.