Open the input file.
Create an output file.

Loop until the end of the input file is reached.
   Read a line from the input file.
   If the line contains the error code you're checking then update the date on that line.
   Write the line to the output file
End loop

Close the output file.
Close the input file.

(Replace the old input file with the new output file, if required.)

    static void processFile(String filename) throws IOException
    {
        // Open the input File.
        File inputFile  = new File(filename);
        BufferedReader input  = new BufferedReader(new FileReader(inputFile));
        // Create the output file.
        File outputFile = new File("temporary file");
        BufferedWriter output = new BufferedWriter(new FileWriter(outputFile));
        
        // Loop until the end of the input file is reached,
        // reading a line of data from the input file each time.
        String lineOfData;
        while ((lineOfData = input.readLine()) != null)
        {
            // If the line contains the error code you're checking 
            // then update the date on that line.
            if (lineOfData.contains("Error_Code :0001"))
            {
                // This is just an example.  You will need to format the date 
                // to suit your needs.
                lineOfData = "Error_Code :0001 occurred at " + (new Date()).toString();
            }
            // Write the line to the output file
            output.append(lineOfData);
        }
        output.close();
        input.close();
 
        // Replace the old input file with the new output file
        inputFile.delete();
        outputFile.renameTo(new File(filename));
    }