Can I upgrade the iPhone 6 Plus from 64 GB storage to 128 GB

You can resolder a bigger chip onto it, then you'd have to do some factory level programming to initialize the chip and write config data into it, which i don't know if it is possible.

UPDATE:

It is possible now, some Chinese repair shops are offering this upgrade:

http://www.askci.com/news/chanye/2015/10...

Based on customer feedback, if all serial number/MAC addresses are rewritten properly, the phone works with no problems. However Activation Lock seems non-functional, you can successfully enable find my iPhone on it, locate it remotely, but after a DFU forced restore, the phone can just be activated again without having to reenter your apple ID.

And whether new updates or activation server changes will improve or destroy the phones are not known.

Yes you can. But the only way is change the flash ic inside logic board and need to reprogram the serial no. same as your existing flash ic serial no.

The only way to upgrade the internal storage is to swap in a logic board from the same model with a larger storage module. As other posters have mentioned, it's possible to unsolder the storage chip from the logic board and solder in a larger one from the same model/generation - if you can find a larger module and do the solder/replacement without wrecking the logic board altogether, or can find someone you can trust to do the work.

This has been true for every model/generation of the iPhone.

However, if the goal is simply to get more mobile storage attached to the iPhone you already have, there are external options for adding extra storage:

1. Mophie has made a line of charger cases called "Space Packs": A protective case with a secondary battery, plus 8-64 GB of built-in flash storage, depending on the model. They've recently had models available for the 5/5S/SE, for the 6/6S and for the 6+/6S+; I've seen used units for 4/4S listed on eBay and my local Craigslist. SanDisk has a similar case called the iXpand, with storage amounts up to 128GB - but only for the iPhone 6/6S. The basic iXpand has no expansion battery, but there's a clip-on battery pack available as an add-on.
2. There are Chinese vendors on eBay who do plastic cases with slots for microSD cards, similar to those used for extra storage in some Android phones. The iPhone models I've seen have a slot that's supposed to accept up to a 256GB card. While the Mophie cases have a secondary battery, the Chinese cases have a Qi wireless charger receiver built in, like the Qi wireless charging system built into the iPhone 8/8+/X. That way, you can lay your phone down on a Qi charger (either a common generic one, or the one that Apple will release this winter) to recharge it. Use the keywords "memory", "expansion" "wireless", "charger" and your cellphone model and you should find something.
3. What I use with my iPhone 6S is a third option: A Lightning-to-USB dongle with built-in flash storage. It has a Lightning connector at one end and a USB-A connector at the other; you can connect it to your phone/iPad or your computer as a thumb drive, and you can use the entire dongle as a super short recharging cable. The flash drive appears as a standard finder volume on a computer, just like any other thumb drive. On an iPhone, you access the external storage through a management app from the manufacturer, which allows you to move files between internal and external storage, or to view/play/edit files stored on the external drive. SanDisk sells a version under the iXpand name, at sizes from 16-256GB. Lexar sells a version called JumpDrive c20i or c25i, in sizes from 16-128GB. A consumer note: Crucial/Micron, Lexar's parent company, has recently sold the Lexar division to a new owner, and Lexar flash devices are going cheap right now. I find that movies stored on the Lexar drive play as smoothly in Lexar's management app as they do in Apple's TV app.

It totally is. I’m starting to think jessbethany has some affiliation with Apple.

Change the disk with larger and transfer the device details to new disk .

https://goo.gl/28zKJQ