You can change the volume of the system sounds only, while leaving your normal notification volume at full. Open the volume panel, either by tapping on one of the volume buttons and tapping the three dots and moving the slider down on the slider with the wrench icon, or by opening settings, sounds and vibration and selecting system sound and moving the slider to lower the volume, or tapping on volume in that same setting and lowering the volume. Most of the time you don't need to hear them anyway I've never had any kind of notification sound for a system update, even when I was awake and everything was at full volume. I am puzzled that the sound for this notification was different than your normal system sound - the only sound that I know of that would be different is the Emergency Alert System. If you have Good Lock/Nice Catch you can turn on sound history and see which app it actually came from and turn off the notifications for that app only.
If you don't have or don't want to download NiceCatch, you can search the app list in the settings - search for software update (make sure "show system apps " is on), tap to open the settings, go to notifications and then notification categories (you must have notification categories turned on), tap on that and the only category will be "general". tap that (you can't turn it off), and select sound and then select "silent" as the sound. Any of these actions will likely solve your issue. My guess is this last one would be your best option for this particular notification. Sorry for all the words, but i wanted you to have all the options so that you can take the action best for you.
