In order to do this, you would need to desolder the flash memory from your iPhone's logic board, then solder on a recycled one from an old iPhone with a larger storage capacity. This would require a TON of soldering skill, as you would have to properly desolder the flash memory from both devices, then resolder the larger one back on to your device.
All in all, I would not recommend this. However, I'm sure there are millions of people out there better at soldering than I, so if you are feeling extremely ambitious, this guide will get the logic board out for you.
Hope this helps!