You need a photo-editing program such as Photoshop, Painshop Pro, Gimp, or something else.
You'll need to open up the asset for Editing in Explorer through CMP, or find the asset in TRS2004 in Explorer, edit the texture files and then you'll have your new reskin. I'd suggest saving the file first to a new version so you don't lose the original.
At one time there was Paintshed, but that program never took off the way it should have and it was always very difficult (kludgy) to use IMO.