I want to upgrade my storage

You cannot upgrade the storage on your phone. It will always have the amount of storage it came with. You can buy space in iCloud, Apple's synching service. See this link...

What's the difference between device storage and iCloud storage? - Apple Support

The only way you can gain space is to delete photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. this video may help...