Upgrade from 32bit to 64bit

Hi

I just built my first gaming rig and a friend came to help me installing windows 7. In my college I have access to a free download, but I did not know how to put it in a pen-drive. So he just installed his own version which is 32bit.

I just need the home premium and having 32bit with 8gb ram doesn't make any sense. I downloaded my copy(home premium 64bit) but the computer doesn't let me install it. I've been reading and it seems that in order to install the 64bit I'll lose all my data(that doesn't matter 'cause I have none).

I want to know (preferably step-by-step) how to install my version of windows 7.