How can I update my iPhone 7 Plus to iOS 17?

You don't. The hardware on the iPhone7 Plus cannot handle iOS 17. Software has always outpaced Hardware ever since the first computer and it will be that way in the future as well. You will need to get a new phone that can handle the software you would like to use.

The iPhone 7 Plus can't even run iOS 16.

If you want to run iOS 17, you will need an iPhone XR or higher model.

I have an iPhone 8 Plus. I found out the hard way that it wouldn’t update to iOS 17 either. I bought a new Apple Watch & when trying to pair it with my phone, it wouldn’t. I Googled it & found out I wouldn’t be able to do it with my 8. In trying to get a new iPhone, it turned out my AT&T account had been hacked by someone getting a new iPhone on my account & made it look like I had upgraded my phone which I hadn’t. Now I am paying for a new phone that I don’t have & I can’t get a new one until that is paid off. It’s been a mess. AT&T won’t do anything about it. 😢

If I had a nickel for every time this question gets asked..... ;-)