How can I update my iPhone to latest iOS version?

Your tag line says that you have an iPhone 6s Plus.

You can update an iPhone 6s Plus to the latest release of iOS 15 – currently iOS 15.8.3.

About iOS 15 Updates - Apple Support

Update your iPhone or iPad - Apple Support

You cannot update an iPhone 6s Plus to iOS 16 or later. For the iPhone 6s Plus, which ran iOS 9 when Apple first introduced it more than nine years ago, iOS 15 is the end of the line.

To update your iPhone 6s Plus to the "latest" iOS version that it can run......

Tap Settings > General > Software Update

If you see a message that your iPhone is already up to date, then your iPhone has gone as far as it can go update wise.

If you want to run the current iOS 18, you will need a new phone. My recommendation would be an iPhone 15 or iPhone 16.

iOS 15 is the end of the line for your 6s

Neither we, just users like you, nor Apple can do you this favor, sorry.

Find out why in this compatibility chart: