This is a fairly simple and straightforward process. First off, select the model. Not the rig but the actual mesh. Next go into the texture tab and go down to where it says Image.
Click on the X to get rid of the texture, and then hit the Open button to open up your new texture. That's it, however, you won't see a change in the 3D viewport because Blender is a poop. Open up the UV Image editor and go into edit mode with your model. Select all of the geometry with A, and select your new texture from the menu.