foreach (CadBaseEntity entity in cadImage.Entities)
{
  if (entity.TypeName == CadEntityTypeName.LWPOLYLINE)
  {
    (entity as CadLwPolyline).ConstantWidth = 2;
  }
}

This can be used for common entities
foreach (var entity in cadImage.Entities)
{
    entity.LineWeight = 24;
}