Why would you want to do that. JailBreak you IPod. But if you want to update for any other reason :
How to update iPod for Windows using Windows Vista, Windows XP or Windows 2000
Download and install the latest version of iTunes. When you have the latest version, use the iTunes menus to check for updates. On Windows choose Help > Check for Updates.
Connect your iPod to your computer using the USB or FireWire cable that came with your iPod and open iTunes if it is not already open.
If you want to update your iPod on a PC, it must be formatted for a PC. If it is not, you will see a message stating that your iPod needs to be reformatted. If you receive this message, refer to Restoring iPod to factory settings before proceeding. If you are unsure of your iPod's format, refer to iPod: How to determine your iPod's disk format.
If iTunes detects that a newer version of iPod software is available for your model, you will be prompted to download and install the new version. If you have already downloaded the newest version of the software, you may be prompted to install it.
Depending on the iPod model and whether you use USB or Firewire to connect to your iPod, you may be asked to leave your iPod attached to the computer for the update to complete, or you may be asked to connect the iPod to the power adapter.
Note: not all iPod models ship with a power adapter. Be sure to follow the onscreen instructions to complete the update.
During the update process, you will notice that an Apple logo and progress bar appear on the iPod display.
Note: this may be difficult to see since the iPod's backlight will be turned off. Once this is completed, you can resume normal use of your iPod. If you notice a different icon on the iPod screen, refer to iPod iconology or what does this picture on my iPod mean? and the follow the related instructions.
How to update iPod for Macintosh
Download and install the latest version of iTunes. When you have the latest version, use the iTunes menus to check for updates. On the Mac choose iTunes > Check for Updates.
Connect your iPod to your Mac using the USB or FireWire cable that came with your iPod and open iTunes if it is not already open.
If you want your iPod to be updated on a Mac, it must be formatted for a Mac. If it is not, you will not be able to update your iPod, only restore it. In this case, refer to Restoring iPod to factory settings before proceeding. If you are unsure of your iPod's format, refer to iPod: How to determine your iPod's disk format.
If iTunes detects that a newer version of iPod software is available for your model, you will be prompted to download and install the new version. If the newest software has already been downloaded you may be prompted to install it.
Depending on the iPod model and whether you use USB or Firewire to connect to your iPod, you may be asked to leave you iPod attached to the computer for the update to complete or you may be asked to connect the iPod to the power adapter. Be sure to follow the onscreen instructions to complete the update
Note: not all iPod models come with a power adapter.
During the update process, you will notice an Apple logo and progress bar on the iPod display
Note: This may be difficult to see since the iPod's backlight will be turned off). Once the update is complete, you can resume normal use of your iPod. If you notice a different icon on the iPod screen refer to iPod iconology or what does this picture on my iPod mean? and follow the related instructions.