I want to update my iPhone

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, you can update to 15.7.5

• If you have an iPhone 8 or higher, you can update to 16.4.1

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

Hello,

You can only update to iOS 12.7.2. See this:

Updating iOS: common issues and solutions - Apple Community

Also see:

Update your iPhone or iPad - Apple Support

Good luck.

iPhone 6s.....which you mentioned in your tagline.....can be updated to iOS 15.7.5

Tap Settings > General > Software Update

You can only update to iOS 12.7.2.

Does not exist.

iPhone 6s......which the OP told us he had......can go to iOS 15.7.5.