How to upgrade iPhone 16 Pro storage from 128GB to 256GB?

It's not possible to upgrade the storage on an iPhone. You'll need buy one with more storage. You can probably sell the one you have and use the money toward one with more storage.

You can't. Sell it, or if you're within the time allowed by the store's return policy, use that.

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There is no way to upgrade your storage short of buying a new phone with larger capacity. Apple phones will always have the storage capacity they came with.