Which software specifically are you trying to run, most 32bit software will automatically work fine on a 64bit OS.
If it does not then good chance you will need to run it in a virtual machine or download a 64bit version.
As for changing to 32bit windows from 64bit, you would have to wipe your system and reinstall windows (32bit). This would limit you to 3gb ram maximum, even if you have more it will not be used. So I wouldn't recommend it these days.
Actually you dont have to buy a 32bit version of windows at all.
Microsoft very nicely left the whole thing wide open, so as long as you use a valid License code it will work for both 32 and 64 bit versions. Also if he bought the retail version of win 7 it comes with both 32 and 64bit disks in the pack.