Cush
usually, you see the download-process in the upper right corner of the screen and after download is finished, the extracting/installation process begins and you also see a message for this in the upper right corner of the screen.
when installing has finished, you get a message in the top middle of the screen, that you have to reboot to finish the update process (you then manually open the main menu and go to reboot).
i would suggest, you do a manual update by downloading the boot.tar.xz from here:
https://updates.batocera.org/rpi3/stable/last/boot.tar.xz
and extract it to the root of your sd-card and replace existing files.
then put the sd-card back into your RPi3 and boot from it.