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. Because a 64-bit operating system can handle a large amount of memory (4GB or more), it can be more  efficient than a 32-bit operating system; a 64-bit system can be more responsive when running several programs at the same time and switching between them frequently.

Most programs designed for a computer running a 32-bit version of Windows will work on a computer running 64-bit versions of Windows. Notable exceptions are many antivirus programs, and some hardware drivers. (Microsoft Website).
You cannot upgrade from a 32-bit system to a 64-bit system without going through the process of reeinstalling Windows operating system.