There is no flash drive just soldered flash memory chips.

Sadly, you'll need to swap out the complete logic board which is quite expensive! Best to sell and buy a new unit with the larger storage.

UPDATE

While the flash chip can be replaced it does take a bit of skill, availability of the chip and the needed programer. This it's not a DIY project for a good 90% of us.

Even still as Tom pointed out there are risks if the upgrade will be stable and its already known some actions don't work correctly.

Given the market its still better to buy a new phone! If you must trade it in or even sell it your self to recover a good part of the costs of a new phone.