Good; they're better off without updating anyway. Hopefully HotSWin will release the updated version later today for interested dataminers, but it won't be worth it for normal people to update.
If your client is prompting you to update, exit your Battle.net launcher (don't leave this running while running an unregistered alpha, anyway, seriously, that's not smart), try renaming StormSwitcher.exe to HeroesoftheStorm.exe and running it again (without moving it, it has to be in Support folder to work), or try renaming the entire folder that your Heroes of the Storm is installed to.
Worst case scenario, you got a bad download. Download from one of the more recent links posted in this thread.