A cron task is an instruction that is performed a a certain moment, as scheduled in a file called crontab.
In order to edit the crontab, do
and edit the line you want, with the time schedule at the beginning.
The format is
minute day hour day_of_month month day_of_week
More info on how to set the schedule here: http://en.wikipedia.org/wiki/Cron
Remember tu reload the configuration, doing
service cron restart
If you would like to see if the cron is being executed, check the default log file (syslog):
grep CRON /var/log/syslog