How do you update an iPhone from itunes on a computer?

To update an iphone using iTunes on a computer, first connect your iphone to the computer using a USB cable. Open iTunes (or Finder on macOS Catalina and later) and select your device when it appears. Click on "Summary" and then choose "Check for Update." If an update is available, click "Download and Update" to begin the process, and follow any on-screen instructions to complete the update.