First question is why do you want to go to a 32 bit Windows 7?
Are you having issues running a 32 bit application?
It doesn't really make any sense to revert to a 32 bit operating system when you really have a better system with the 64 bit. The bus is wider and will address more RAM also.
If you don't mind give us reason why you want to change back to a 32 bit system.
If you're really insistent on installing a 32 bit Home Premium then go to the following link and download the ISO image file and burn it to a DVD blank disc. Use ImgBurn, its free and burn no faster than 4X to get an accurate burn.
Use the product key that came with your 64 bit system, look for the COA label it should be there.
Windows 7 ISO – Official Direct Download Links