Centos Crontab

systemctl start crond.service

systemctl stop crond.service

systemctl restart crond.service

reference refe

Updating Cron Centos

 nano /var/spool/cron/root

then run

crontab -e

this will update crontab 

(press "ESC + :" )
to Save "wq!" exit w/o changes q!

then run

systemctl restart crond.service

to restart cron

crontab

Other Instructions here