This is a paid asset, but now you can download Terrain Adjust for FREE, Please keep in mind this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Terrain Adjust v1.1.1
This tool helps you adjust terrain height data to 3D objects.
Sometimes during environment design, this brush can come in handy to adjust the height of the terrain to the elements. The brush sampling position and angle from colliders when you dragging on it. This tool is a lightweight package that won’t mess up your project.
Useful for blending roads / rocks to the terrain, or create ramps.
Features
Adjust terrain height and slope
Smooth edges
Customizable brush size / falloff / spacing / etc.
Works as continuous brush
Different blending types
Dockable window
Key shortcuts
Undo / Redo support
Lightweight
This package does NOT include the sample assets shown in the screenshots.
Limitations
Need Collider on target object
Works only in editor