Well it's not entirely clear what exactly what you are trying to do but i'll try and list different scenarios.
If you want to make adjustments to a vanilla race's texture then you just have to open the textures.bsa in data folder with for example bsa browser utility in obmm and extract the texture you want. Then you can open it in photoshop or gimp if you have .dds plugin installed. Then once you have made your tweaks you just pop that texture into your data folder in an appropriate folder. Then all you have to do is open the race in cs and assign that texture.
If you have a a completely new texture you obviously just have to pop it in data folder and assign the texture in cs.
If you have never opened photoshop or cs you obviously need a more detailed guide.