The Global Functions module will allow you to set security in bulk. Let's have a look:
1. Click 
-> Configuration -> Global Functions.
2. Click on the Catalogue tab.
3. Find the "Security" Global Functions and double click on the function you would like to run.
For this article, I have selected "Set Security by Sublocation" This allows me to select a Sublocation and then set a security for all items with that Sublocation.
4. Enter in the Sublocation you wish to update the security on and click 
, located at the bottom.
The Global Function module will then show you a list of items:
Here you can select items you wish to be changed OR you can leave them all deselected and the system will change them all.
5. Click 
.
The Change function will appear now.
6. Enter the new Security you want the items from the previous image to have.
7. Click 
.