I have previously made a small guide on how to change the texture of an object using NifSkope. I just recently found out over at the Dark creations forum that you can also use the Creation kit it self to do this. The advantage of using the Creation kit is that you will not make a new nif file. But on the other hand you can't as easily share with others the re-textured object as a resource. Another disadvantage using the Creation kit is that you can't (as far as I know) adjust the UV-map of the model to fit the texture better. I often adjust the UV-map, for example by scaling and rotation so the texture will fit nicely on different models, without having to make several different textures.
Lets take an example:
I want the cliffs in the game to match the textures we are using in the conifer forest we are making. I used NifSkope to set the matching rock textures as I have to scale the UV-Map so the rock texture would be the same size as the ground texture.
First of if you don't already have this, you need to have a texture set for the texture you wish to use. You can find these under Miscellaneous and Texture set.
Many of the cliffs also have dirt textures on flatter surfaces, and these I want to change to the moss ground texture I have around the cliff.
Double click on the cliff in the object window to edit it. Make sure to give it a new name, otherwise you will change all the cliffs of this type in the game. Press the Edit new to the Model path.