Hi @Onyx_Lee,
Which tileset are you using?
There’s no way to import a 3D Tiles tileset as a landscape and sculp it, unfortunately. But if you’re using Cesium World Terrain, and you have better heightmap data for your area, you can upload that to Cesium ion and create a local overlay on top of Cesium World Terrain. Then you can access the combination from Cesium for Unreal as a single tileset.
Another possibility - once again assuming you’re using Cesium World Terrain - is to export your area using Cesium Clips. Then you can edit it in a 3D modeling package and re-upload it to Cesium ion to create a tileset from it. You can then add that to Cesium for Unreal and put a Bing Maps raster overlay on it.