cPanel offers you to schedule backup and retain backup of server and accounts. But if you need to run backup manually, you can do it from SSH. Login to your server as root via SSH and run following command.
For cPanel new Backup system
If the backup is up-to-date, you can force to run backup using below command
For cPanel Legacy backup system
If the backup is up-to-date and you wish to run backups anyway, use: