This update is coming out of the band (meaning it's not part of the second Tuesday of the month cycle of update) because it's an important update - there are reports of malicious activities already using the vulnerability covered by this update.
In my experience Windows Server 2008 64 bit do not require a restart. All other versions will require a reboot.
Just visit http://windowsupdate.microsoft.com to do it. You'll find more information here.
Other related posts:
Windows 8 Mail app not hyperlinking emails
Are we seeing the death of Windows RT?
Windows 8 Consumer Preview
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