Windows 7 Upgrade Windows 7 32 bit to 64 bit?

Hi folks, this is my first time posting so I hope someone can help! Now I know you've probably heard this topic a hundred times before, but please stick with me as I've been googling this for a few hours now with no avail!

Currently, I have Windows 7 32 bit installed on my Dell Studio 15 laptop. I originally had Windows Vista, but upgraded to Windows 7 Professional 32 bit for free during an event on my college campus. My laptop is however, 64 bit capable, so I'd like to upgrade to 64 bit. I realize this entails a clean install and I'm ok with that, however I have no idea of how to do that! I don't have any upgrade discs because the upgrade was done for me at the event. I have located my product key by downloading an application online that found it for me. Now, is there a (relatively) pain-free way for me to install Windows 7 64 bit on my capable computer, even though its 32 bit right now and I lack installation discs? If anyone could help explain this to me, the simpler the better, I'd be much obliged!

Thanks so much!