is latest ypdate of 15.4 available on inphone

iPhone 6S is currently supported up to 15.5. Please see: iPhone models compatible with iOS 15.5 - Apple Support

However iOS 16, being released later this year, will not support the iPhone 6S. If this matters you may want to consider upgrading your iPhone model. iOS 16 Preview - Apple

The iOS version for a 6s is 15.5.

An iPhone 6 cannot update past iOS 12.5.5. What phone do you have?

It says you have an iPhone 6 (not a 6s).

iOS 12.5.5 is the last version for an iPhone 6.