that would be a very hard thing to tell you, since it has so much into it, it is hard to explain here in a couple of lines...

this is a site which is devoted completely to Reverse Engineering, meaning altering an executable without recompiling the source code. (most of the time even without the possesion of the source code).

this is done by the use of debuggers / disassemblers, hex-editors and you could also use a resource editor for some parts.

i would suggest you start reading as much as you can from the site mentioned above and google for the words "Reverse Engineering".

i just feel like i should warn you, it is a subject which will take a long time mastering it, you can not do it just in an evening or even a few months .

hope this helps