How can I update my iPhone 7plus to ios16

Your iPhone 7 Plus is too limited for iOS 16 and later, as correctly mentioned above.

Microsoft supports the three most recent iOS versions: iOS 16, 17, and 18.

It’s likely iOS 16 support won’t last the year, and anything that can run iOS 17 can also run iOS 18.

You’ll want an iPhone XR, iPhone XS, iPhone SE 2nd, or newer, if you want to use an iPhone for this.

These models can all run iOS 18.

Sorry, but you CANNOT update an iPhone 7 to iOS 16 or higher. It can be updated to iOS 15.8.3 and no higher. It is too old and lacks the hardware required to be updated further. If you need to download apps requiring a higher iOS version, then it's time to retire you very old (it is 9 years old at this point) iPhone and buy a new or newer, iPhone.