Label.StyleName API allows to get or set the Entity object's style name and we can use this to update PipeLabel Style.
Here is a C# .NET code snippet :
PipeLabel pipeLbl = ts.GetObject(pipeLblID, OpenMode.ForWrite) asPipeLabel;
// Label.StyleName Property -> Gets or sets the Entity object's style name.
ed.WriteMessage("\nStyle Name Before Change : " + pipeLbl.StyleName.ToString());
// Now update the Style
pipeLbl.StyleName = "2D Length - Total Span";
ed.WriteMessage("\nStyle Name After Change : " + pipeLbl.StyleName.ToString());
In the following screenshot we can see a particular Label Style being used for Pipe Object in Civil 3D.
And here we see by updating the StyleName how the Label style is changed :
