This short blogs show you how to update your Windows 7 since the Wanna Cry Ransomware is attacking badly at this time.
There are pretty of links out there but I remember this official one work for me.
Windows Update is taking an unusually long time to scan and install updates
April 2015 servicing stack update for Windows 7 and Windows Server 2008 R2
July 2016 update rollup for Windows 7 SP1 and Windows Server 2008 R2 SP1