Upgrading your OS from 32bit to 64bit is as easy as installing the Operating System. You will need to buy a 64bit version of your operating system, then I advise making complete a full backup of all your data (images, music, downloads, etc) and then perform a fresh install of the new 64bit OS.
Please be aware that some program manufacturers DON'T offer 64bit versions of there software, 99.9% do but please be aware of this.
Unfortionatly the only way to upgrade from 32bit to 64bit, is a fresh install of Windows.