In Graal Editor first enable "Baddies, NPCs, Chests -> Show Terrain Grid" if you haven't. You'll see a lot of yellow + and numbers representing the height of the terrain. It sounds like you already found this, but I'm just saying it to be sure.
Anyways, once you do that you can drag the + up and down to change the height. If you hold ctrl down while doing so it will snap the terrain to the nearest whole number.
You can also change the height of the terrain via the setz() command:
PHP Code:
setz(x,y,width,height,a,b,c,d); sets z of x,y with a,b,c,d as corner heights
(use updateterrain to save)
In which you will have to use updateterrain() immediately afterwards to have the changes take effect. This is apparently all done clientside so I don't think this is a legit way to permanently manipulate terrain. You'll probably have to ask Tig for more information regarding this.