My iPhone 6s is not updating from version 12,it’s not giving me any update option

Hey there ,

are you sure your iPhone is iPhone 6s not 6 ?

to be sure Go to Settings > General > about > model name!

and i want to ask you what do you see when you go to Settings > General > software update?

If it is running iOS 12.5.5, it can not be an iPhone 6S.

Instead, it has to be an iPhone 5S, 6 or 6 Plus. Neither can be updated further.

Hello ~ Take a look here also:

Identify your iPhone model - Apple Support

~Katana-San~