You showed no effort by showing us your code, you need to reference autocad, which is the correct app to open .dwg, to "manipulate it as you state.
Another solution is to use DWG TrueView -
View and convert 2D and 3D DWG files \ Free (as they state)[
^]
Your code will then start with -
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.Windows;
using Autodesk.AutoCAD.Interop;
public class DWGViewer
{
}