Lightroom will respect the device settings regional date format. If you want the appearance to change, please see iOS Settings > General > Language & Region, and choose the region and calendar settings that show the date formatting as you prefer.
If you want to modify the date metadata, the Lightroom Mobile app currently cannot do that. The Lightroom and Lightroom Classic desktop apps can do this--however these are paid apps that offer free use only during a 7-day Trial period... Before trying that, look for info on the internet to see if the Lightroom desktop apps would provide the functionality you need. Example: https://www.lightroomqueen.com/change-exif-dates/