How to reverse numbers to alphabet and alphabet to numbers in a string?, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            
            
            string content="D93AK0F5I";
            
            
            
            Console.WriteLine(content);
            Console.WriteLine(convert(content));
        }
        
        static string convert(string input)
        {
            var chars = "0abcdefghijklmnopqrstuvwxyz";
            return string.Join("", 
                               input.Select(
                                   c => char.IsDigit(c) ? 
                                   chars[int.Parse(c.ToString())].ToString() : 
                                   (chars.IndexOf(char.ToLowerInvariant(c))).ToString())
                               );
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,13 sec, cpu time: 0,11 sec, average memory usage: 14 Mb, average nr of threads: 3

How to reverse numbers to alphabet and alphabet to numbers in a string...