In order to reset the overdue notice level for an invoice, go to the invoice, then go to "Overdue notices". Once there, you'll see all your previous payment reminders and overdue notices. To change the overdue notice level, click on "Actions" - "Delete overdue notice" for an overdue notice.
The overdue notice level will automatically be reduced from to .
To give you a better overview, you can follow the entire process in the GIF below.