You can't just simply change it automatically. You'll need to reinstall Windows with a copy of the 32-bit os. You will also lose all your files. But you can back them up on another device like an external hdd/sdd
I don't really see the point in changing from 64-bit to 32-bit unless there's an application that you really need that doesn't support 64.
64 bit operating systems use the full amount of ram, while 32 bit doesn't.
For example, if I had 4GB of ram, it would likely only use 2 gb of ram.
Performance and speeds are also lower than 64-bit.
With 64 bit, more things are supported, and there are more speeds.
I upgraded from 32-bit to 64 bit not too long ago and I really do notice the difference.