Tap on day you wish to change in the calendar, then tap on the Day Properties icon (3rd from the right at the top).
You will see a Reminders button at the top of this page. Tapping on it will show the reminder options for that day.

To override the default reminder settings (the ones set for the shift type), tap the “Custom reminder for this day” option to show all the options you can change.

Any changes you make here only affect this specific day