how do i change my full iphone device storage to my empty icloud storage

1.

Try to free up iPhone device storage see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

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

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

2.

What's the difference between device storage and iCloud storage?

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

If you turn On iCloud you get 5 GB iCloud free storage.

Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.

If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

See the following Apple articles:

How to use iCloud on iPhone

How to use iCloud on iPhone – Apple Support (UK)

Manage your iCloud storage

Manage your iCloud storage - Apple Support

iCloud storage is not an extension of your device, there is no way to store items from your phone only in iCloud.