From Reminders Help:
- Click the reminder’s Inspector button (i).Or, double-click the reminder.
- Select "On a Day."
- Click the date to select or enter a different date.
- Click the time to enter a different time.
- If you want to be notified more than once, choose an option from the Repeat pop-up menu.For information about repeating reminders, see this help topic: About recurring reminders
- Click Done.
For additional options, select "Reminders Help" from its Help menu.
Normally, if you hover your mouse in the vicinity of the reminder of interest ("Replace..."), a little button with an "i" in it will appear. You can click that to configure the reminder. You should also be able to double-click on the Reminder (double-click in the little box with the text "Replace...", but not on the text) .