terrain decal

how to change texture of terrain, i want the terrain seems to be damaged like war games, how to use decal but must have the shape of terrain

I suppose it depends on how you create terrain.
Common are the Terrain Scene Manager (available up to Mogre 1.7, removed in 1., the new Ogre Terrain Component or loaded meshes (bad for large terrain).

By a quick search I found following links:
http://www.ogre3d.org/tikiwiki/Projective+Decals
http://www.ogre3d.org/tikiwiki/OgreDecal
http://blog.csdn.net/pizi0475/article/details/6715621

I know that the Ogre scene creator/editor Artifex Terra uses terrain decals for the terrain tools.
Artifex Terra uses the Editable Terrain Manager (ETM), which is also available for an older Mogre version.
So you can search inside of the sources of Artifex Terra or ETM to find the related code. (I suppose this is time consuming.)
http://www.ogre3d.org/tikiwiki/Artifex+Terra
http://www.ogre3d.org/tikiwiki/ETM

Additionally you can search/ask in the Ogre main forum and try to port the related C++ Ogre code to C# Mogre code.

Unfortunately I can't help you more.