Digging around on the Symantec web site, I found the answer you seek:

To schedule a scan for Norton AntiVirus 2005, do the following steps:

1. Start Norton AntiVirus. If Norton AntiVirus is installed as part of Norton Internet Security or Norton SystemWorks, then start that program.
2. In the left pane, click Norton AntiVirus if it is listed. If it is not listed, go on to the next step.
3. In the left pane, click Scan for Viruses.
4. In the right pane, click the first icon to the right of the custom scan that you want to schedule.
5. Click New and then fill in the Schedule Task, Start time, and Schedule Task Daily fields, and then click OK.

That's how to create a new schedule, so I am presuming that when you get to the area where you'd add new schedules, you'll also see the existing schedules that you have and be able to click on them to modify them to your needs.

You can learn more about the Norton AntiVirus scheduler on their site, at service.symantec.com/SUPPORT.