no, you can't change to 32 bit.
if you want to use 32 bit os, you will have to do a fresh install of the 32 bit os using the 32 bit installation disk. all the 64 bit data will be gone. 32 bit os cannot run 64 bit software. if you have 64 bit programs, you'll have to use 32 bit versions of them (if available).
make sure you back up your important data beforehand.
imo 64 bit os is better because it can run 32 bit software as well.