The date format configuration is linked to the language that you use.
If you are seeing the month first (MM/DD/YYYY) you are most likely using English (US) and would need to change to English (UK) should you wish to have the day first (DD/MM/YYYY).
It's worth mentioning that changing this would also change the day that your calendar starts on, but only on the business dashboard, not for the customers.
If you are looking to change the starting day of the week for your customers' calendars, see here.
To change language, please scroll to the bottom of your TeamUp page >> locate the language selector >> select English (UK) >> Save.
You should now notice that the date format has changed to DD/MM/YYYY.