Hover your mouse over the terrain in the Scene view and you left mouse to paint on it. Currently, there are 3 paint mode supported in this version: Raise, Lower and Smooth.
Raise: this is the default mode, surface vertices will be move up and not exceed Surface Max Height property. In this mode, brush cursor will have cyan color.
Lower: you have to hold Ctrl key to use this brush, surface vertices will be move down and not exceed Base Height property. In this mode, brush cursor will have red color.
Smooth: you have to hold Shift to use this brush, this will apply a Laplacian smooth function to the vertices in range. In this mode, brush cursor will have yellow color.
Note: Vertex displacement using brush (hand painted method) is an additive and un-normalize operation. That means it is not clamped by the height map and painting data at a position will not change when you resize terrain dimensions.
Note: When painting, the terrain will display a quick preview version, which may have incorrect color, some case it disables flat shading due to large dimension. Don't worry, everything will be correct when you release the left mouse.