I want update this phone how may I do that

On your iPhone 7 Plus.......

Make sure that you have a good Wi-Fi connection and fully charged battery

Tap Settings > General > Software Update to see if an update is available for your phone.

Unfortunately, the iPhone 7 Plus is getting quite long in the tooth, so it cannot run recent iOS versions like iOS 16 or the current iOS 17, nor will it be able to run the new iOS 18 version that will be released in the next few weeks.

You may already be running into some situations where an App needs a newer iOS version to run, so I imagine that it will not be too long before you will need to think about a a new phone.

An 8 year old iPhone 7 cannot be updated higher than 15.8.3

If you need a newer version of iOS you will have to get a newer model phone:

• iPhone XR, XS, SE 2nd or 3rd edition, iPhone 11 and newer: You can update to iOS 17.6.1
• iPhone 8, and iPhone X you can update to 16.7.10