Need help with modifying monsters

[pesupulveri]

Hello there everyone!
My problem is that I want to change monster's model and textures so that I can put my custom human-like face into it. I have in few days achieved only so little. Grunt and Brute both have issues with their morbid faces so I can't insert directly the new face on them.
I have tried to modify them with Blender but the UV mapping screws me over. And Amnesia model editor crashed when I tried to import the mesh. I have tried to search everywhere for a simple guide on how to modify models and textures. I only need the face over the monster's original face... like a mask. Simple headshape would be perfect for me.

So alternatively I have and idea to modify Alexander to an enemy, with same animations that Grunts uses. I think it's possible. But I don't get the modified Alexander model to work with model editor. I have exported it as a .dae file. Hope someone can help me... I am confused here

And if you can, please try to keep it simple. English is not my native language and I am somewhat new with this. But I'll ask if I don't get something