How to update ios new version?

The OP says that he has an iPhone 7 Plus. That phone cannot run – and will not be offered – iOS 16 or higher.

That iPhone 7 Plus should be able to run the latest release of iOS 15 – currently iOS 15.8.1.

About iOS 15 Updates - Apple Support

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community

The latest iOS version is 17.3.1.

You will need an iPhone XR or a newer model to be able to update to iOS 17.3.1

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Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone. 

• Your iPhone must have a fast Wi-Fi connection or a 5G cellular connection. You cannot update over a cellular hotspot
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 12GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.