Hi Sam. It really depends on which sim you have. Also if you wish to repaint the defaults or 3rd party aircraft. The texture formats vary in each case. You will almost certainly find DXTBmp (mentioned above) useful to convert the textures for editing & back again to the appropriate format when they're finished. Then you will need a decent paint program/graphics editor to do the actual repainting. If you don't have anything suitable there are various excellent freeware graphics editors. Which one you choose will be your own preference.
After that the rest is down to your skill & experience with the graphics editor & editing the textures.
