You asked... and you got it!!! :)

First you need to download the Firmware from the Manufacturer's website. Make sure you download the right file and you get the Model and version number of your router on the bottom of the device.

After downloading save the files on the desktop or any folder and follow these steps:
Note: Make sure your router is plugged into your computer.

1. open a web browser such as Internet Explorer or Netscape and enter the IP address of your router (given by your manufacturer)
2. Enter your username (admin) and your password (leave the field blank). Click OK to enter the web configuration for the device.
3. You would find a firmware upgrade screen either in the tools menu or administration menu (Depends on the router brand you have)
4. Click on the Browse button and browse to the file that you have downloaded from the website, Click OK or Apply.

If you get a "Page cannot be displayed" message or any similar error, just close the browser, reopen it and start the process again.

Hope this takes care of your problem..!!