The easiest way, imo, is to dump the textures yourself and then edit them in a graphics program. Usually, you can vectorize a 2D image, scale it, and then rasterize it to get a bitmap image. Using something like Inkscape or Adobe Illustrator, it doesn't take much artistical skill. There are even program out there that can apply scaling filters like HQ2X or SuperEagle on images, so you could run your dumped texture through that.
Once you have the dumped textures and you've edited them, all you have to do is load them as custom textures. Then you'll see the images you edited show up in the game.
Once you have the dumped textures and you've edited them, all you have to do is load them as custom textures. Then you'll see the images you edited show up in the game.