using System;

using System.Xml.Linq;

using System.Xml;

​

public class Program

{

    public static void Main()

    {

        var xml = @"<Root>

  <Marys_basket numFruits=""2"">

     <Fruit name=""Mango""/>

     <Fruit name=""Papaya""/>

  </Marys_basket>

  <Jons_basket numFruits=""0"" />

  <Bobs_basket numFruits=""1""> 

     <Fruit name=""Apple""/>

  </Bobs_basket>

</Root>";

        var doc = XDocument.Parse(xml);

        XElement basket = doc.Root.Element("Marys_basket");

        basket.FirstAttribute.SetValue((int)basket.FirstAttribute + 1);

        Console.WriteLine(doc.ToString());

    }

}