You can start it with systemctl start podman-auto-update.service It’ll auto update daily at 00:00.
Be aware you need to enable and start podman-auto-update.timer for this to work automatically (ie systemctl enable --now podman-auto-update.timer), this command will just update the images once only.
I don’t think this works for non-system podman images, so you’d have to do systemctl --user enable --now podman-auto-update.timer for each user.