using System;

using HtmlAgilityPack;

​

​

public class Program

{

    public static void Main()

    {

        var htmlDoc = new HtmlDocument();

        htmlDoc.LoadHtml("<p onclick =\"alert('abc')\" style =\"color: black\"> text </p>");

        var pNode = htmlDoc.DocumentNode.SelectSingleNode("//p");

        pNode.Attributes.Remove("onclick"); 

        Console.WriteLine(pNode.OuterHtml);

    }

}