How can I upgrade my iPhone7 plus

You can update an iPhone 7 Plus to the latest release of iOS 15, currently iOS 15.8.3.

About iOS 15 Updates - Apple Support

You cannot update an iPhone 7 Plus to iOS 16 or higher. If you need something higher than iOS 15, you will need to get a newer iPhone, preferably one that is compatible with iOS 18 and that also supports 5G cellular networks.

If your iPhone 7 Plus is in good condition, Apple might offer a small amount of trade-in credit for it. (It doesn't look like they accept any iPhone older than than an iPhone 7 Plus, except for recycling.)

Apple Trade In - Apple

If you get a new iPhone, these links may be useful.

For the new one: Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

For the old one: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you are referring to upgrading your iOS, that won’t be possible. Your iPhone 7 Plus has reached the end of its software updates support for the latest iOS operating system, and cannot be updated past iOS 15.8.3.

If you are referring to upgrading your iPhone, you can gladly do so. You can purchase the latest iPhone at any Apple Store or Apple Authorized Retailer. By the way, many carriers also offer deals for the latest iPhone when you trade in your old iPhone.

By the way, I recently purchased the iPhone 16 Pro Max and I am loving it.

Purchase the latest iPhone ⇢ iPhone 16 and iPhone 16 Plus - Apple