update my iphone 6s

I'm guessing that you have an iPhone 6 not a 6S. Your stats show you are on iOS 12 which is the highest you can go with a 6. If by chance you do have a 6S, follow the instructions below.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S or newer, you can update to 15.0.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204

I want to upgrade ios12 for apple phone 6s to ios15

I want to upgrade ios12 for apple phone 6s to ios15

You don't have a 6s. You have a 6 or a 6 plus. 12.5.5 is the end of the line.