You are going to make a big configuration change, hardware upgrade or mass updating user files on your cpanel services. Making a backup is the first thing do in such cases, and you want to do it before your scheduled backup.
Cpanel does not offer making instant backups just like plesk. You will need ssh access with root to do it. After you log in as root
It will prompt that backup process has been started in background along with log file location which is usually on path /usr/local/cpanel/logs/cpbackup/