Not sure I understand what you asking.
Do you mean you have some land some where and you want to remove it?
Or you have a terrain type of grass, or farm or something and you want to change it to some thing else.
If you want to remove some land you have placed in the terrain, you can just draw water over
it in the water tab. Instead of selecting the erase water select draw water, and the depth to deep.
If it's a tile type like grass and you want it to be farm, then under the terrain tab select the type you want and paint over
the one you want to replace.

midi