In the data editor you can find 'Terrain Deformation' objects which basically allow you to make squares or circles of height changes by placing those doodads which allows for random heights though I believe you cannot adjust cliffs ingame (as said by blizzard anyway) perhaps you could make a cliff illusion but that would be pretty complicated, you'd need to make a doodad that could work as a cliff, have terrain deformations placed in such a way it actually looks like a cliff when you put the doodad next to it and also place pathing blockers (could be dummy units) to prevent units going over the cliffs on the ground. The real cliffs ingame are also illusions but integrated into the game system (just hide cliffs and you'll see the terrain is just heightened like you could do normally aswell).
I think you should be able to find a terrain deformation tutorial somewhere here, I used one a long time ago. Try the search function.
In the data editor you can find 'Terrain Deformation' objects which basically allow you to make squares or circles of height changes by placing those doodads which allows for random heights though I believe you cannot adjust cliffs ingame (as said by blizzard anyway) perhaps you could make a cliff illusion but that would be pretty complicated, you'd need to make a doodad that could work as a cliff, have terrain deformations placed in such a way it actually looks like a cliff when you put the doodad next to it and also place pathing blockers (could be dummy units) to prevent units going over the cliffs on the ground. The real cliffs ingame are also illusions but integrated into the game system (just hide cliffs and you'll see the terrain is just heightened like you could do normally aswell).
I think you should be able to find a terrain deformation tutorial somewhere here, I used one a long time ago. Try the search function.