The default way would be to install the package unattendedupgrade which will install security updates on your system every day.

Depending on how pihole is installed, this may not upgrade it. I personally have it installed with docker and uses watchtower for updates, but the risk with this mechanism is that it can break things.

I've set a cron job on the raspberry pi at my parents' house. It's a debian based system with only PiHole running so I just pihole -up and apt update && upgrade every month or so.

I run this on a cron to auto update PiHole:

    #!/bin/sh

    LOG_FILE=/var/log/update_pihole.log

    echo "Starting upate" >> $LOG_FILE
    date >> $LOG_FILE
    pihole -up >> $LOG_FILE
    pihole -g >> $LOG_FILE

    exit 0

I've been using: https://www.reddit.com/r/pihole/comments/bgzvem/how_to_healt... for upgrading + healthchecks

However note that the developers of PiHole do not recommend automated upgrades, or at least be aware of the implications