My iPhone 12 storeg no going to update

1.

mobile Storeg not using 128 gb

That is your iPhone device storage, the iPhone 12 storage 64GB, 128GB or 256GB. 

You can 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.

useing only 5gb how can I use 128gb

See -> 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.

If you need more, see this Apple article: Upgrade to iCloud+

Upgrade to iCloud+ - Apple Support

See also: iCloud+ plans and pricing

iCloud+ plans and pricing - Apple Support

If you have upgraded iCloud storage, it would take about 1 to 3 business days in order to process.

Once your request for iCloud+ has been activated, you should receive an invoice in your email box.