The official line on BC textures is to go into your texturesets folder, make a copy of the default folder, and then edit the bitmaps.
However, everytime you restart BC, it forgets which texture set you have chosen to use, and so you can waste time making surface maps with the wrong textures.
So my advice is make a backup of your default folder, and then just edit the default folder itself, which saves you a lot of hassle.