Grab your hex editor and drop your desired model your wanting to use different skin.
I use HxD.
I use a dday trooper.
Use msh viewer to confirm the changes.
Heres my files. We want to get a model to use a new skin by hexing.
The red is the copy of the exact same model. Thats the one ill be hexing.
Heres the skin were going to change (rep_inf_ep3trooper) skin name)
Open your hex editor and drop you model in there. ddaytrooper_49d
On the right side with all the ugly data, your looking for your texture name.
Once you find it, simply click on it and type away the new name. But do not extend the name. They must be same lenght.
Save the file
Now use msh viewer for confirm the changes. New skin
The model might hold multiple names like the ddaytrooper. So if you want to change the dday gear textures, simply hex the gear texture names as well and save. That's all there is to it.