If you already own the 32bit version and you simply want to switch that machine to 64bit then it's perfectly legal to download the 64bit version and do a complete reinstall for free. You can get the 64-bit version
HERE and you simply install it using your existing product key. Note: that you MUST download the same version as you already own, so in order to install 64-bit Home premium you must already own the 32-bit version of Home Premium. Also make sure you have your product key written down before you format and install the 64-bit version; your product key is normally on the original disk or on a sticker attached to the computer, but if not you can find it from the existing install by using a free software tool called
SIW (but it must be done prior to reformatting!!!).
I recommend backing up your user accounts and their data first, using Windows Easy transfer. You will also need all your software disks because they will need reinstalled after the fresh format and install.