1. Download the tiles you want and import them with import manager.
2. Change the paths of the imported files to the terrain file paths you want to replace.
For example if you want to replace Ashenvale dirt texture then change the texture path of your imported file to
TerrainArt/Ashenvale/Ashen_Dirt.blp
You can check the file paths with War 3 Viewer for example. They are all in TerrainArt folder.
When you use Ashenvale dirt tile in your map, it will show up as your improted texture.