To change a build parts mesh you go to the build part blueprint of the build part you want to change like for example the BP_WoodFoundation, you can find all the build parts in the Blueprints/BuildParts folder. Once you are in the blueprint go to its viewport and select the StaticMesh (Inherited) and in the details you can set the Static Mesh setting.
Pivot Point – Things to remember when replacing the exiting build part meshes, you mesh should have its pivot point in the centre for all the build parts except doors, which should have its pivot point on the edge of the mesh that it should rotate from see the included wood door for an example.
Size – The mesh should be the same size or close to the same size as the default meshes unless you are replacing all the grid build parts e.g. Walls, Floors, Foundation and stairs with new sizes, if not then you build parts wont fit together well in game. If you are changing the sizes of the meshes you may also need to change the grid sizes and locations. To change a grid location select the grid component inside the build part blueprint and simple move it in the viewport. To change its size check out the guide linked below for more info on gird components.
Build Collision – For more info on the collision of the build part check out the Adding Furniture video linked below, I explain how build collision works and how to change it and this information applies to all build parts not just furniture.
Ground Checkers – These are also explained in the video linked below.