Editing player model

I’ve a question about editing a player model. I’ve searched on the Internet but I can’t find what I want.

I’ve a A model and a B model (downloaded on garrysmod.org).
I like the A’s head so I want to replace B’s head. (originally, without modifications, theses heads are the same).
And I want to change something on the B model (it’s an aperture scientist but I want to remove the “Aperture” on his white coat).

So I would like to be explained how I can do this task (softwares, how to do etc…).
Thank you !

EDIT : I can also give the folders with the files of the 2 models for someone who can do it for me. (Send me a private message.)

It’s called headhacking and skinning, there are tons of tutorials.

Thank you for the answer.
I’ve found a tutorial but I’ve got a problem with Studiocompiler.
When I choose a model and the output directory, Studiocompiler doesn’t work (and I have an error message).