why my iphone cannot to be update version

Not certain why your iPhone may have issues or trouble in using

an available update to its iOS. Does it show one version available?

In general terms this article covers several details; maybe read it first.

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

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

"If you can't see the update on your device, use your computer to update

your device manually. Learn how to update your device manually."

if you're using a Mac with macOS Catalina or later, or if you're using

a Mac with macOS Mojave, or earlier or a Windows PC."

• How to update to iOS 15 or iPadOS 15 - Official Apple Support

//support.apple.com/ios/update

"Make sure your device is plugged in and connected to the Internet with Wi-Fi. Then follow these steps:"

1. Go to Settings > General > Software Update.
2. You may see two software update options available. You can choose to stay on iOS and iPadOS 14 and still get important security updates. To update to iOS and iPadOS 15, choose that option.
3. Tap Install Now.

..If your iPhone cannot use the latest iOS15./15.4, it may use iOS14 or before..

There are more ideas across support articles which may further help.