How can i get the latest ios

Go to Settings > General > About

What does it say for iPhone Model and Software Version?

You need an iPhone 6s/6s Plus or newer in order to run iOS 15..

Supported iPhone models - Apple Support

If your iPhone is really an iPhone 6 Plus, then you cannot upgrade beyond iOS 12

have you read thru this support article for updating?

Update your iPhone, iPad, or iPod touch - Apple Support

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..

📲The latest iOS version for your iPhone 6s Plus is iOS 15.5.

📵If you cannot update your iPhone 6s plus, try checking your storage.

🔧To update, go to Settings -> General -> Software Updates.

Start here: settings, general, software update. What does it show? An iPhone 6s Plus can update to iOS 15.5.