Make sure the new file does not have a second zip extension. It is a common error. Simply rename the file "update" without the quotes or zip extension. If that fails, try another sd card if you have one.
depending on how you have your computer configured...if you have it set to show file extensions (ie. .zip, .exe, .jpg) then you should NOT take away the .zip because it is part of the file extension..
I'm assuming your computer is set up like this....LEAVE THE .ZIP extension in there and just rename everything else to "update"....leaving you with the file "update.zip"