In order to upgrade cores in Zx-UNO BIOS, rename "COREX.ZX1" with the core slot number you want, example: CORE8.ZX1, copy it to SD root folder, then enter in BIOS (F2 at boot), select upgrade tab, and select slot you wanted (8 in this example), and select SD upgrade. Wait until done, put name to the core slot when prompted. Then reboot and press Caps Lock at boot. Select core to boot, and voila!