The terms 32-bit and 64-bit refer to the way a computer's processor (also called a CPU), handles information. The 64-bit version of Windows handles large amounts of random access memory (RAM) more effectively than a 32-bit system. If you are currently running a 32-bit version of Windows, you can only perform an upgrade to another 32-bit version of Windows. Similarly, if you are running a 64-bit version of Windows, you can only perform an upgrade to another 64-bit version of Windows.
If you want to move from a 32-bit version of Windows to a 64-bit version of Windows, you'll need to back up your files and then perform a Custom installation of the 64-bit version of Windows. For more information, see Installing and reinstalling Windows.