As linked just above  - quick way is to edit the using f12 via browser under your advanced settings to set the 500 value for frequency to your over clock (see https://bitcointalk.org/index.php?topic=3509006.msg45020727#msg4502072)  However another quick way depending on how your farm may be setup is just to ssh in and edit the file in /config/cgminer.conf and edit the frequency by hand there in that file, and then reboot the antminer (type 'reboot' if you are in ssh).

For those not familiar with SSH, check out a program calledWinSCP or  PuttY.  WinSCP is even easier, it lets you navigate and edit files too), and just connect using the default user 'root' default password is 'admin' to your port for SSH/SCP (port 22).  You can then if using PuttY edit the file using VI (good luck if you are new to unix, sorry but google is your friend).  WinSCP is much easier to remotely access and edit files on the antminer.  Perhaps easier than using developer mode (f12) in your browser to hand edit frequency.

Again the file is located in folder    /config    and is named cgminer.conf  - you may find it easier to manage things there on all your different antminers anyway than browser access if you have lots to manage.

p.s. I use 675 Frequency and get about 14 K up to as high as 15.92 K output - varies between my different Z9 minis for the same setting (all batch 2's) and even unstable on others.  Still adjusting to fine the best stable one.

edit: 3 of my units from 'china' (fedex delivery) clock at 675 fine and stable, produce 14 to 15.9 K output.  My other 3 units that came from Hong Kong shipping origin (DHL) all dropped out a chip after about 10-15 minutes, at 675 Clock Frequency.  at 650 Clock its stable for those 3 units producing about 13.9 K up to about 14.4 K.  Arrived within a day of each other this week batch #2 - the theory that different batches, and chip quality can differ would hold true.   Why hong kong vs china origin ship units differ I dont know.  Serial numbers on the 3 from honk kong end in 914, 919, and 932.  The China serial numbers are wildly different for letters and numbers, again all 3 arrived and shipped together from china.  Temps on all are 68C to 70C for what its worth on the over clock.  (Its hot out here and using open-air system with fans).