How can I purchase more storage?

Before you buy more iCloud Storage, make sure you understand that having more space in the cloud, will NOT increase the storage on your iPhone.

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

There is no way to increase the physical storage an an iPhone after purchase.

Upgrade your iCloud storage plan -

Upgrade to iCloud+ - Apple Support

I need upgrade my storage

Refer to lobsterghost1's post, make sure you understand the difference between your phone's storage and iCloud storage.