How can I update my iPhone 6s software?

ketan110 wrote:

Phone update

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support If the above Apple articles do not help, please post back with relevant information. 

You can update an iPhone 6s to the latest release of iOS 15.

You cannot update an iPhone 6s to iOS 16 or higher, if that is what you are asking about. Once you are running the latest release of iOS 15, the phone will not offer you further updates (unless Apple releases another iOS 15 update).