how do i update my iphone 5

The iPhone 5 was discontinued 10 years ago. The best that the iPhone 5 can do is iOS 10.3.4. No more updates are available after that for the iPhone 5.

To run the current iOS 16, you need an iPhone 8 or higher model.

the Apple iPhone 5 is way way too old to be updated and I may be wrong but I am pretty sure it can only use until IOS 10 and if you want to use the current IOS 16, you will have to buy a new phone.