Step 1: Open Crontab on Raspberry Pi
sudo crontab -e
Step 2: Add Cronjob to Raspberry Pi
0 3 * * * root /usr/bin/apt-get update && /usr/bin/apt-get upgrade -q -y
sudo crontab -e
0 3 * * * root /usr/bin/apt-get update && /usr/bin/apt-get upgrade -q -y