How can I update my 16gb I phone 6s

• 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 15.0.2

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

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

Use iTunes/Finder to update to iOS 15.0.2 which requires approx 8.2 GB of space on your device