I finally got it working THIS WAY:

   public void Main()
        {
    string fileText;
    try
    {
      fileText = System.IO.File.ReadAllText(Dts.Variables["FullFilePath"].Value.ToString());
      fileText = fileText.Replace("WTOTA", "TOTAL");
      System.IO.File.WriteAllText(Dts.Variables["FullFilePath"].Value.ToString(), fileText);

      Dts.TaskResult = (int)ScriptResults.Success;
    }
    catch (Exception ex)
    {
      Dts.TaskResult = (int)ScriptResults.Failure;
      throw ex;
    }
    finally
    {
      fileText = null;
    }

    Dts.TaskResult = (int)ScriptResults.Success;
        }