modify dll file in asp.net without source code

Question

I have dll file in my project without source code of dll file now I want some changes that file how I change it..

 

Please help me.

 

this is the code of dll I want change date in this code

namespace ESPLHelper
{
    using System;
    public sealed class Config
    {
        private const string version = "EH.L.01";
        private const string ServerDetails = "Endeavour";
        private const string ExpireDate = "2020-12-31";
        public const char ColumnSeparator = '~';
        public const char RowSeparator = '`';
        public const char TableSeparator = '^';
        private Config()
        {
        }
        public static bool Authentication()
        {
            try
            {
                return (Convert.ToDateTime("2020-12-31") >=DateTime.Today.Date);
            }
            catch
            {
                return true;
            }
        }
    }
}