For any map, a developer is going to need to customize the combo of textures to get just the right look and feel for your map. Don't worry, you can get the job done in just a few steps.
1) On the menu bar, go to Map -> Map Textures... Here you can convert the use of a specific texture on the map to another texture anywhere that texture was used. For now.... we want to click Modify.
2) Right click in the top left pane and click "Add Object ..."
3) Next you will need to fill out the details on your new terrain set. You can choose the terrain set you also want to inherit from if you're looking to just change a few things.
4) After clicking Ok look in the right pane, left column for a field that reads Textures - Blend. If you double-click the value you will then see the window that will allow you to customize which textures are available in your texture set. Remember, each map can only have one texture set and each set can only have eight textures.