macbook air storage upgrade

What year/model is your Macbook Air? Your hard drive/SSD is larger than 50 GB. You must be referring to iCloud storage? You would need to purchase that through Apple. Here is an explanation:

https://support.apple.com/en-us/HT201318

Hi,

First of all, which model your MacBook Air is?(you can see Apple menu > About this Mac...).

You can't upgrade 2018 or later model since SSD is soldered on logic board.

secondly, you should take back up data before upgrade SSD.

If you are in Tokyo, upgrade SSD is available at Akiba-Kan in Akihabara.

You are charged extra, though they will replace SSD for you.

https://www.akibakan.com/guide/shoplist/index.html

https://www.akibakan.com/hdd-ssd/internal_ssd/internal_ssd_air/

Anyway, contact at 03-3255-8786

Or Make Genius Bar reservation at Apple Store Ginza.

https://www.apple.com/jp/retail/geniusbar/

https://www.apple.com/jp/retail/ginza/

Contact at 03-5159-8200

You can pay by cash, credit card or debit card.

It tells you how in the link I provided. It will be charged to your Apple ID.