Can't be done, once you install a 64-bit edition of Windows the only way to switch to a 32-bit edition is to make a clean install using a 32-bit edition installation disk. If you need to run a 32-bit application that won't run on Windows 7 64-bit I think you should try to figure it out using the compatibility options within Windows first or try installing Windows XP Mode ( more info here). If you need it for compatibility with your hardware... well, I think you should really consider an upgrade already.
You'd need to do a fresh reinstall from the DVD. The license (COA) will install either 32 or 64 bit versions of the particular OS level (if it's a 7 pro license, it'll work for either pro 32 bit or pro 64 bit).
As other members on the forum have specified, if one wishes to move from Windows 7 64-bit to Windows 7 32-bit a clean, or "custom", install will indeed be required. Please remember to first backup any and all important data as a “custom” installation is considered to be a clean installation. Any data currently on the machine can be lost when performing such an installation. To learn more about custom installations for Windows 7, please see the following resource: