In order to install the improved textures in the folder with your mod you need to: download them in the workshop, or an => alternative link <=[drive.google.com] if you have problems with the installation through the workshop. In your case, it will be much easier to download a folder with textures via Google Drive, then open it with any archiver (I use total commander).
Next, go to the folder with your mod (by default Native)
c:\Program Files (x86)\Steam\steamapps\common\Mount Blade Warband\Modules\Native\
and copy the Textures folder to it, which is located in the archive.
Start the game and enjoy the new textures!