How to increase storage capacity of an iPhone 6 w/16GB to 32 GB?

You cannot. The physical storage capacity of your iPhone will forever remain what it was at the time of purchase. In your example, your 16GB storage capacity will always remain the same.

Axel F.

bruie Said:

"storage capacity: I have an iPhone 6 w/16GB. How can I increase the storage to 32GB?”

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Perhaps it is Time for a New iPhone?:

An Phone 6 is obsolete, with recent updates and iOS versions. It does not meet the hardware required for the iOS. So, consider getting a new iPhone — one with enough internal storage to suit and meet your needs.