You need a tool like NFS-TexEd. You can get it here:https://nfs-tools.blogspot.com.ar/p/downloads.html
You can use that to open files that contain the game's textures. In the game's folder called TRACKS you'll find files like STREAML4… and some letter at the end. The one that ends with A has the main city textures (used for most races and for free roam). The other ones have the drag race map textures (I think), the URL track, the stadium track, the airport.
The skies are in the file: LOC4DYNTEX
Stuff like the license plate, etc. are in GLOBAL/globalb.lzc I think.
Don't forget to run TexEd as administrator. You can use it to export textures and re-import them after editing.
Now for editing, first you need a way to open the textures as they are in DDS formats. I think there's a photoshop plugin thing that lets you do it. Otherwise I use a program called DXTbmp. It lets you save them as BMP or TGA and then you can use your favorite graphics editor to edit them, then use DXTbmp again to save them as a DDS again.
There's different types of DDS formats, the game uses mostly dds 1 and dds 3. The "1" is for images with no transparency (for example a wall) and the "3" is for the ones that have transparency like smoke. Transparency is achieved by having a "alpha" image which is a black and white image that comes with the main image in the texture. The black parts are the most transparent ones.
After you saved as DDS in the same format as the texture you extracted you can re-import it to the game's texture file it was in using NFS Tex Ed again and save.
You will notice tha the textures get a bit grainy and blurry after saving but that's normal I guess.
Notice that some texture mods have huge textures, this is because they use something else called Tex Mod. This is a program that loads textures externally, not by editing the game's files. When you edit the game's files you can only stay in their original size. Except for the case of LOC4DYNTEX (the one with the skies) because there's a method to get bigger textures there using the NFS U2 Mod Tools (extract every single file in it, then make the ones you want to be bigger, make sure the file names are correct, then create a whole new LOC4DYNTEX file instead of editing the original using the texture generator thing for cars that comes with the mod tools).