How do I update my iPhone 6s

#1. Shut off the VPN.

#2. READ the information in that screenshot.

"This update cannot be installed because it requires at

least 4.17 GB of available storage during installation.

You can make more storage available by deleting items

in iPhone Storage, or connect your iPhone to a computer

and use iTunes to install the update. Learn more..."

Settings -> General -> Software Update.

A 6s is updatable to 15.4.1 (the current iOS)

A 6 can only go to 12.5.5.

Settings>General>Software Update.

What do you see when you go there?

Settings - general - software update.