I can't update my iPhone 7

If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support

Please also describe a more complete description of the issue such as what you are trying to do, where it fails to work as expected, and critically, any error messages you see.

Available, or total? 16 GB is almost certainly stuffed full, so yes, you’ll have issues pending offloading whatever you can.

If available, remove any beta, remove any add-on VPN client app, restart your Wi-Fi router, force a restart, and try again.

Use a computer to update: Update your iPhone or iPad - Apple Support

The iPhone 7 doesn't seem to support iOS 16, You can view more information here: iOS 16 and iPadOS 16 are compatible with these devices - Apple Support