Just keep in mind that after changing textures you'll often need to restart the editor for things to work properly. It's easiest determining at least more or less what you plan to work with, inserting those 8 textures, and then restarting once.
Go to the Data Editor and click on the green cross tab to open the Terrain Types tab if it already isn't open. Then select the name of the terrain you're using and double click the Textures - Blend field to replace or delete the tiles that you're no longer using.
If your tile doesn't appear in the brush list (while using the Terrain Editor module) even after you've replaced it, try changing your current layer from Terrain to Units or something else, and then go back to Terrain again. The editor has a habit of not updating the brushes list in real time. If that still doesn't work, then try closing the map and reopening it again.
Keep in mind that you should only have to restart the editor if you decided to import custom textures that replaced an existing one.
I recommend spending an hour or so on a blank map seeing what tiles from different tilesets go together and which ones fit your needs. It is possible to switch later, but you'll have to remove all your terrain and start all over in the process.