Retexturing is kind of easy to accomplish: Find the textures you want to replace (eg. with the NWNExplorer1.69), extract them (that explorer can extract/save files for you to other locations - if available, use the DDS version of the texture as a base). Edit them and place them in the NWN override folder (or in a HAK and load that HAK into your module).
It can be as easy as that, but the results you'll get will be bad. If you want to make a proper override you have to make sure some of the textures match in tone. For exterior textures you have to make sure that they don't have too much detail if they are used repeatedly on rockface and ledges or on the ground. Especially grass and rock textures with a lot of detail don't work well in NWN. Less detail is better here.
Next make sure the textures aren't too photo like or too picture like. Also too saturated textures look completely out of place with other content, use them and you might get an area that looks good without placeables, but it all falls apart once you add placeables with different colours.
Especially too green tree foliage doesn't work. When using foliage textures and sometimes flower textures the most commonly made mistake is that the colours on them are too uniform. The same green on every leaf or red on every petal will make your areas look like a child's picture where the sky is blue, clouds are white and grass is the same kind of green everywhere.
If the tileset uses grass on different textures, make sure the grass textures match, like the main grass texture has to match the grassrims and other textures where grass is blended with dirt (like the anthill texture), rock or gravel.
For interior tilesets make sure you don't use too many different materials on the same tile. If a wooden doorframe has a different texture than the wooden floor which is different from the wooden roof which is different from the wooden pillars, you are using too many different types of wood. The textures can vary in appearance, but the colours should match so that at least some of them appear to be made from the same wood. The richer the inhabitants of a house the more different kinds of wood you can use for it.
All this means that you can make a texture override without editing the textures you use - but it won't look good! To get a good texture override just replacing textures isn't enough. You also have to edit them to match tones, to blend grass with gravel, etc.
