How to buy more storage

What storage are you referring to? Are you referring to iCloud storage? If so, here is how to do that:

1. Open Settings
2. Click on Apple ID (at the top of the page)
3. Click on iCloud
4. Click on manage account storage
5. Choose Change Storage Plan

Those are the steps for getting more iCloud storage, but if you were referring to the internal storage of your iPhone, then sorry, the iPhone's main storage is not upgradeable.

An iPhone's internal storage isn't upgradable.

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