Increase memory iPhone 6 plus
Can I increase memory in my iPhone 6 Plus
From the way I understand it ( haven't done it ) you would have to have your HDD NAND IC removed by micro soldering, have the information copied (programmed to a new larger sized HDD NAND IC and Micro Soldered back on to increase memory on iphone. Very risky though to do with a chance of bricking your phone. Or you can simply purchase larger icloud storage amount and sync your files to online for access.
It depends where you live, if you go live or go to china, this sort of thing can be done in a hour at most shops. Outside China its done less so, especially on an old phone like a 6 plus too. In the west a business would charge more than buying a new phone to do it, where as in China its not particularly expensive and can be done pretty quickly.