using System;

using System.Xml.Linq;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var xml = @"<?xml version=""1.0"" encoding=""utf-8""?>

<EAs>

  <EA MagicNo=""000"">

    <Date>2022-07-21</Date>

    <Symbol>AAA</Symbol>

  </EA>

  <EA MagicNo=""111"">

    <Date>2022-07-22</Date>

    <Symbol></Symbol>

  </EA>

</EAs>

";

        XDocument doc = XDocument.Parse(xml);

​

        var nodes = doc.Elements("EAs")

            .SelectMany(e => e.Elements("EA"))

            .SelectMany(e => e.Elements("Symbol"))

            .Where(e => e.Value == "");

        foreach (var node in nodes)

        {

            node.Value = "BBB";

        }

​

        // save xml

        Console.WriteLine(doc);

    }

}