you cant "upgrade" nor would you want to upgrade is the worst thing ever
backup and important files (docs emails favourites photos music etc) to another pc/hdd/ext hdd then install 64-bit
if you dont have a 64 bit dics you can download and burn one online and use your existing product key (100% legal) http://www.mydigitallife.info/windows-7-iso-x86-and-x64-official-direct-download-links-ultimate-professional-and-home-premium/