1) you have a good downloaded zip file.
2) the USB stick is formatted FAT or FAT32.
3) the file is not update.zip.zip (can easily happen when using windows).
One or more of those three have stopped more people updating than anything else. Number three catches more people than it should because the default in windows is ti NOT show the extension so if you name the file update.zip windows will add .zip without telling you. That will never work.