Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Terrain Quality Manager Free.
Terrain Quality Manager v1.1
Terrain Quality Manager allows you to change the resolution of Heightmap, Detailmap, Alphamap and Basemap, without losing data.
If you need to increase the quality of terrain, or optimize terrain for better performance, with this tool you can do it.
• Unity v5.2 – Unity 2019.x.
• Allows you to change resolution of Heightmap, Detailmap, Alphamap, Basemap and Resolution Per Patch, without losing data;
• Works for single terrain, all terrains in scene, all terrains in project;
• Allows you to scale up and scale down terrain maps;
• Seamless result, when working with multiple terrains;
• Very easy to use.