Look to the top right corner of the UI web page. You should see a power button icon up there. You can shut down, restart, or log out from the UI from there.
You can also reboot or shut down from the serial console, whichever is easier.
You can also tell the UPS to turn off as part of the shutdown process, to help prolong the life of the battery and to limit the beeping. However, the UPS has to have the built-in capability to do so.
Some BIOS', IIRC, also allow the machine to auto-boot when power is restored, ditto the UPS. So I'd look into that.