In AutoCAD Civil 3D user interface (UI), in the 'Drawing Settings' dialog, at 'Units and Zone' tab, we can select a geographic coordinate system from the available list and apply the same to the current DWG file.
Here is a C# .NET code snippet which demonstrates how to do the same using .NET API -
//start a transaction
using (Transaction trans = db.TransactionManager.StartTransaction())
{
using (Autodesk.AutoCAD.ApplicationServices.DocumentLock locker = AcadApp.DocumentManager.MdiActiveDocument.LockDocument())
{
// perform any document / database modifications here
SettingsDrawing settingsDWG = civilDoc.Settings.DrawingSettings;
//settingsDWG.UnitZoneSettings.CoordinateSystemCode = "LL84";
settingsDWG.UnitZoneSettings.CoordinateSystemCode = "FL83-EF";
ed.WriteMessage("\nCoordinateSystemCode = " + settingsDWG.UnitZoneSettings.CoordinateSystemCode.ToString());
}
trans.Commit();
}