how to install IOS in IPHONE 8

You already have an iOS on your phone, if you didn't, your phone wouldn't work. If you mean the latest version for an iPhone 8 which is iOS 16.7.2 -

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support