You "have been sent an update". What does that mean? A friend of you attached the file to an email and sent it to you? Or something else?
I'd ask the sender to resend it. Usually, it means something got lost during the download. Can happen. Then most often the second or third time go OK.
Of course, it's possible that it already was invalid at the source. Then the sender should make a new original. But that's an exception.
I have just been sent an update which I tried, but failed, to download.
The update is called AVG Anti-Virus Free Edition 2013 2013.0.2667 and my system says that it is not a valid Win32 application.
How do we overcome this?