Is there a way to make the update notifier check for updates daily (like you could in Software & Updates) from the command line or by editing a file? I checked the GSettings schemas, but didn't find any keys that change values if I change the frequency in the graphical menu, so I don't think it's any of those.
Update Notifier already checks daily.
It's an apt systemd timer. It runs at a different random time each day.
In this example, you can see that apt-daily (which runs
sudo apt update) ran 4:24 ago, and will run again in 1:37.
$ systemctl list-timers apt-daily.timer NEXT LEFT LAST PASSED UNIT ACTIVATES Tue 2020-10-13 22:15:09 CDT 1h 37min left Tue 2020-10-13 16:12:52 CDT 4h 24min ago apt-daily.timer apt-daily.service
Randomization: The randomization evens out the load on the mirrors. Those mirrors are storage and bandwidth volunteered by organizations worldwide -- be very nice to them.
The random time is set by a line in
/lib/systemd/system/apt-daily.timer. You can disable it and set a specific daily time, but remember to be a good neighbor: Don't hammer mirrors by picking a common time. If possible, leave the randomization alone. It works.
If you simply want to update your system, you can place the following in
/etc/crontab and set to run as root user:
apt update && apt upgrade -y
If you want to use a GUI application, you can simply call
update-manager, which will prompt for a password when run.
user535733's answer is obviously more accurate, but I'll leave this here in case someone wants to manually call the GUI updater.