We've prepared firmware 18.104.22.16813 for USW. This has been pushed to several recent controller versions. Please see below for the changelog and links to the firmware binaries.
Firmware changes from 3.9.3:
[USW] Add SNMPv3 support.*
[USW] Add priority queue support.
[USW] Mostly fix a bug which causes a switch to stop passing traffic every provision.**
[USL2] Improve fan levels.
[USXG] Fix an issue with SFP speed autodetection.
[USXG] Improve handling of IPv6 frames.
[USXG] Fix an issue with RJ45 SFP speed detection.
[USXG] Improve SFP+ DAC compatibility.
[HW] Add IPv6 management support.*
[HW] Improve event notification on device upgrade.
[SEC] Fix CVE-2017-14106.
*controller support either varies or is pending (only found in 5.7.x+).
**This provisioning bug affects all prior versions of 3.9.x. This fixes most known issues, although we are still investigating another possible case. If you experience issues, please revert to 3.8.6 as that's the last known best firmware for UniFi Switch (all models).
Links are all HTTPS. This assumes your device(s) are currently running at least 3.7.51, and that the device's date is properly set (i.e. it can reach a NTP server). If you encounter issues with the upgrade, or your device isn't on a minimum supported firmware, then you can change the link to HTTP before performing the upgrade.