Q: We updated yosemite 10.10.5 with latest security update (from Sept 1, 2016).   Wifi is now constantly disconnecting.   Is anyone else seeing this?

NOPE!

A few suggestions:

If you have not already done so, repair permissions & restart your computer afterwards.

System Preferences>Network

Click the Assist Me button.

In the next window that pops up, click the Diagnostic button & do the necessary.

Macintosh HD>System>Library>Coreservices>Applications Double click on the Wireless Diagnostic.app & follow the onscreen instructions.

https://support.apple.com/en-us/HT202663 About Wireless Diagnostics

Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer. 

See these basic networking KB Articles:  http://support.apple.com/kb/HT1401 AirPort troubleshooting guide

http://support.apple.com/kb/HT4628 Wi-Fi: How to troubleshoot Wi-Fi connectivity

http://support.apple.com/kb/HT2712 Using network locations in Mac OS X

User Tip:  https://discussions.apple.com/docs/DOC-6411 Most common Wifi issues

Manually provided DNS server addresses are higher priority than DHCP's

http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting

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What to do when you can't connect to the Internet

Also, run the Airport Utility app which is located inside the Utilities folder.