[iMore Question]

have a 6S plus apple iphone with 16gb need to increase to 128gb's

[Gtech21]

You would have to switch phones. If you are within the 2 week time frame of owning it you can return it and get the size you need. Otherwise you will have to try and sell it and then buy he 128 gb version.

The inside storage of the iPhone is not able to be changed.

[ieddie]

You can't do it unless you change out the logic board. Would be easier to just go purchase the 128gb.

[KingGrimlock]

You might want to try out the SanDisk iXPAND. It's $100. I don't know how it works, though.

[iphone5s]

^

OP That's actually a decent solution if you're at the point of no return. If you can return it do so and pony up the few hundred dollars. As previously pointed out here there are no other reasonable solutions.