You can export the heightmap of your landscape from unreal engine. Then open it up in a image editing program that can work with 16 bit images (like gimp) and change the heightmap (lower the contrast, at best in a way that you know how much the range has changed as you would need to properly increase the z-scale of your landscape to compensate for the lower contrast in the heightmap). Then import that new heightmap again and change the landscape z-scale accordingly.