The triangles the terrains are made of are way too big to make smooth mountains etc. is there anyway to subdivide it ? or am I doing something wrong ? thanks
Use a bigger heightmap with smaller "meters per pixel" value. 2048x2048 pixel heightmap works well, 4096 is also possible but may cause problems. It's always a compromise between map size and resolution. If you want to change an existing map, I think the only way is to export the heightmap and blendmaps, resize them and import again.