You need to unpack the games resources:-
Open Nexus(From the SDK) - set the asset directory to be C:\Program Files (x86)\Steam\steamapps\common\sid meier's civilization.
In the top menu select Nexus > Script Browser > Extract PAK Files. This will unpack all the assets to the directories ...common\sid meier's civilization v\resource(create a shortcut to this folder on your desktop, name it CiV Vanilla Resources). There are three folders, Common, DX9 and dx9_low. The 3d art dds's are all in the DX9 folder with no subfolders, the fsxsml's, gr2's and fxsxml's are all in the Common folder with no subfolders.
Once you have done that, create a new mod and import all the terrain dds's you require into it, set them to VFS = true, edit as you see fit. There are lots of them split into regions(Europe, Africa, Asia ect).
Edit: They are layered too, so each tile of say grasslands in game would used 3 or 4 textures, grass itself, shadows, lighting ect.