Instructions
How to Modify an Xbox 360 Using a USB Drive to Install System Updates
Insert your USB drive into your PC.
Download the system update file from the Xbox Support website (see reference). Make sure to save the file on your computer using the original file name.
Unzip the downloaded update file.
Copy the file contents from the folder unzipped during Step 3 onto your USB drive.
Remove the USB drive from your computer, ensuring that you safely remove the device using the method provided by your operating system.
Turn off your Xbox console and insert the USB drive into the Xbox system. Power on the console, and the update will begin automatically. Select "Yes" when prompted by the console, and let the console finish the update. When the system returns to the dashboard, the update is completed. Note that depending on the update, the console may restart itself during the process of installing the system update.