Before going further, here are some key points to be aware of:

• If you are currently running a32-bit version of Windows (regardless of whether it's XP, Vista, Windows 7, or Windows 8), then you cannot upgrade directly to 64-bit Windows-- instead you will need to do a clean install of 64-bit Windows 7 or 64-bit Windows 8.  
• A clean install means your files, settings, and installed programs will not be preserved automatically during the installation process. 
• Instead, you will need to backup your files and locate your program install materials and then restore or re-install them manually after installing the 64-bit version of Windows 7 or the 64-bit version of Windows 8.
• If you purchased a retail version of Windows 7 or Windows 8, then a 64-bit version will be included; your key will unlock both.  If your computer came with Windows 7 or Windows 8, on the other hand, then you may or may not have a 64-bit version as part of the restore and install discs that are typically included with your computer; check the discs that came with your computer or contact your computer's manufacturer to be sure.  If you don't have this option, visit the Microsoft Store.
• Your computer must meet our minimum system requirements.  As a first step, we would suggest you run the Windows Upgrade Advisor.
• Here are Microsoft's answers to common questions about 64-bit Windows and the full details on installing Windows 7 and upgrading to Windows 8.