To access the Terrain painting tools, click on a Terrain object in the Hierarchy window and open an Inspector window. In the Inspector, click the Paint Terrain (paintbrush) icon to reveal the list of Terrain tools.
The Terrain component provides six distinct tools:
You can also create your own custom Terrain tools. For more information about this, see API documentation on TerrainAPI.TerrainPaintTool, and see Unity’s GitHub repository for Terrain Tools.
2019–10–22 Page amended
Updated screenshot to match the new UI and added the Paint Holes tool.