You are able to edit scheduled Reminders through the Web Portal before they have been sent. This allows you to update the Reminder content or change the scheduled date and time.
In the Web Portal go to Admin > Reminder Logs > Select the Edit arrow next to the Reminder > Make the necessary changes > Schedule.
Alternatively, you can edit Reminders directly through the Contact Card.
In the Web Portal go to CRM > All Contacts > Select the Contact > Reminders > Future tab > Select the Edit arrow next to the Reminder > Make the necessary changes > Schedule.