How its' possible to change from 32 bit to 64 bit Processor for MacMini?

What model Mini do yo have?

This may answer your question:

https://en.wikipedia.org/wiki/Mac_Mini

You probably have a Core Solo or Core Duo model. You can upgrade the CPU to a 64-bit Core 2 Duo in the T7200-T7600 family.

iFixit and OWC provide easy-to-follow, step-by-step guides. It's a delicate procedure, but not overly difficult. I'm all thumbs, and I have done it twice.

Cheers!