Imagine it this way......

The texture you are looking at in photoshop is a sheet of paper, and the 3D model itself is sitting there in front of you, carved from a piece of flawless Greek marble....

You cut out pieces of the paper, and stick them on to the marble sculpture to give it colour. Since this is high-tech stuff...your paper is stretchy, and you can use the same piece more than once!

Thats what 'UV Mapping' is. It's assigning bits of the texture image to the 3D mesh to give it colour. As explained above...to save space, the same bits are used more than once to save space, and allow each piece to be bigger. This is the art of good texture mapping. Getting as much as possible into as little a space as possible.

Now...if you want to paint a new skin

1) Find out what skin the model is actually using
2) Re-paint it in Photoshop...BUT DO NOT CHANGE THE PLACES WHERE THE PARTS ARE LOCATED. You cannot change this is a paint program...you need the correct 3D modelling program to do this.

Use the old skin as a guide and you shouldn't go far wrong.