how to upgrade iphone 16gb to 32 gb

The hardware memory within iPhones is not upgradable.

The best that you will be able to do is to manage the use of the memory that you have by removing content and apps that is not critical to have on the device, or offloading some of the leaa critical apps/content to iCloud.

This article is mainly about checking storage use, but also includes some links to articles about optimisation at its end.

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

...or you could look at external storage that plugs into the bottom of your device if you don’t want or need to upgrade and can manage moving the files around.

https://www.macworld.co.uk/feature/iphone/best-external-storage-drives-for-iphon e-ipad-3579792/

If you Goggle for iPhone/iPad external storage you will find plenty of options which are based on flash storage that some can also plug into your computer as well.