You'll need to make sure that you update the FMC FIRST before updating your devices.
For Snort, you'll looking to update the VDB (Vulnerability Database) or the Intrusion Rules... I always get the two mixed up--one updates Snort and the other are the Talos updates). Usually after updating, the update will restart Snort. So expect an interruption to your service.
You can download the VDB directly from the Support Site or via the FMC's download update link (
System > Updates > Product Updates). But you should update the FMC before doing the previous step.
The best guide out there is the following: https://www.cisco.com/c/en/us/td/docs/security/firepower/60/configuration/guide/fpmc-config-guide-v60/System_Software_Updates.pdf
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