I wuold like to konw how to work with unity brush set the heightfield data. and update the resulting to the houdini engine API.
is there a way to do this?

You can edit the generated terrain with Unity terrain brushes, then use that as input for generating height field in Houdini. See https://www.sidefx.com/docs/unity/_terrain.html#Terrain_Input [www.sidefx.com]

You can also create a Input Node asset via the Houdini Engine menu in Unity (HoudiniEngine > New Input Asset). Then use a gameobject with Unity terrain component in the input field.