It may be impossible. I was under the impression that phones are designed in such weird ways to try to pack as much stuff into the given volume as possible. But, that could just as possibly be an excuse for not having replaceable/upgradable parts and thus forcing new purchases. Idk. If you like the idea, you should support them (https://www.thunderclap.it/projects/2931-phonebloks). It's free, but requires a FB, Twitter, or G+.