How can I upgrade my iCloud?

You can upgrade your iCloud storage on your iPhone by opening Settings, tapping on your name at the top of Settings, then iCloud, then Manage Storage. Upgrade your iCloud storage plan - Apple Support

If you're reading this on your iPhone with iOS 14 or newer, you can click this link to upgrade your storage.

Read and follow the article linked above.

Go to Settings, tap on your name, then iCloud/Manage Storage/Change Storage Plan.