WIthout your distribution's name/version, exact help is a little tough. I'm using mandrake 9.1, and the cron settings are in subdirectories in /etc. /etc/cron.d/ and so on. Switch to the appropriate directory, then 'cat filename' to read the cron settings. Use your editor or "easy" system configuration GUI thingy to edit them.
Oh, you have to be root, obviously.