1. Open the file with NifTools.
2. Press Right click on the texture, Go to the tab Texture and click Export.
3. Open up the exported TGA file in a image editor that can edit TGA files.
4. Edit the texture. When done, save it. If asked for settings, keep the default ones and click OK.
5. In NIFTools, right click on the texture again, go to Texture and then click Import.
6. And then click Embed.
7. When you imported your new texture, NIFTools will created a new line called "NiPixelData". Right click on it, Press the tab Block and then click on Remove.
8. Go to File > Save As. or press Ctrl + S and save it.
9. Select the file of the world.img and then right click it, and rename it with some name for example you can just add "_Orig" to the real name.
10. Press action tab and click add, Add your file and test the game.