How can I upgrade my iCloud for more storage on my phone

To be clear you cannot upgrade the physical storage on your iPhone. That's a set up (64GB, 128GB, or 256GB).

However, you can easily purchase more iCloud storage and tell your iPhone to rely on that for things like Messages and Photos and free up space on your iPhone for other things (like apps).

This shows you how to pick a paid iCloud storage plan:

https://support.apple.com/en-us/HT201318

You don't have more storage on your iPhone if you buy more iCloud storage.

iCloud storage is not the same as storage on your iPhone.

You can’t buy more storage on your iPhone itself but here are two links that can help you to free up storage on your iPhone:

https://support.apple.com/en-us/HT201656

Optimizing photo storage on your iPhone can help a lot

https://support.apple.com/en-us/HT205703