OK fine, maybe you've set your mind on that and noone can stop you. Here's what has to be done:
It likely uses a flash chip on the main board to store stuff, you will have to find a 64 GB chip (or depending how its set up, 2 32 GB, etc) that matches up with the existing chip perfectly, both electronically and physically.
You will have to desolder the existing chip and replace it with the new one, assuming its easily done and hopefully doesn't need special equipment. On the unlikelyhood it uses a hidden microSD card to store stuff, congrats, you job is almost done (a la HD7 upgrading). BUT I don't think that's the case with any Lumia.
And say you do get it all onboard and haven't bricked the 920 yet from messing up a solder job or otherwise rendering the memory unusable.
There's now no software on that new chip, so you'll have to figure out how to talk to the currently useless phone and put Windows Phone and even lower level code on there, assuming its all located on that one memory chip and not ROM elsewhere. I don't think we have access to those kind of tools.
And say you succeed in all of those. well does the phone recognize all 64 GB? or will it assume that only 32 GB is available since there is no 64 GB 920?
that's the gist of it, quite the undertaking. good luck!