there are two ways.

1) you can look up the model's VTF file (it's texture, in the materials folder). use a program called VTF edit to export the texture as a tga file, then use gimp or something to repaint it. then just import it back in.

or

2) import the full model into blender. then from there export the UV map. (basically the flattened out version of the model). and paint over that in gimp/photoshop, etc. and make a vtf out of it.

both processes are a little more involved then what i'm describing, but thats the general gist of it.