Do you know where the flag DDS is? Find that file and open it with DXTBmp (freeware). That program shows the main image in the main window, and the alpha image in a smaller window to the right.
Double-click on the alpha image to open it in MS Paint (or Paint Shop Pro, etc.). Then re-colour the image black, save in Paint, and in DXTBmp use the refresh button for the Alpha window.
Save.