WordprocessingDocument Class

public class WordprocessingDocument : OpenXmlPackage

using System;
using System.Linq;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;

namespace WordProcessingEx
{
    class Program
    {
        static void Main(string[] args)
        {
            // Apply the Heading 3 style to a paragraph. 
            string fileName = @"C:\Users\Public\Documents\WordProcessingEx.docx";
            using ( WordprocessingDocument myDocument = WordprocessingDocument.Open(fileName, true))
            {
                // Get the first paragraph.
                Paragraph p = myDocument.MainDocumentPart.Document.Body.Elements<Paragraph>().First();

                // If the paragraph has no ParagraphProperties object, create a new one.
                if ( p.Elements<ParagraphProperties>().Count() == 0 )
                    p.PrependChild<ParagraphProperties>(new ParagraphProperties());

                // Get the ParagraphProperties element of the paragraph.
                ParagraphProperties pPr = p.Elements<ParagraphProperties>().First();

                // Set the value of ParagraphStyleId to "Heading3".
                pPr.ParagraphStyleId = new ParagraphStyleId() { Val = "Heading3" };
            }
            Console.WriteLine("All done. Press a key.");
            Console.ReadKey();
        }
   }
}