subscribe to our Youtube
As @TRIGGERACTIVE said, existing rules could be changed using crontab -l and crontab -e.
Also, you should be able to edit the crontab values by editing the actual periodic reboot configuration file.
Just use vi command to edit /etc/config/periodic_reboot
After you've done with the editing, execute /etc/init.d/periodic_reboot restart
And that's it, your crontab values and WebUI values will change according to the changes made in the configuration file.
I learnt that editing /etc/config/periodic_reboot and then executing /etc/init.d/periodic_reboot restart is necessary to ensure the crontab rules do persist across device reboots.