Hi,

To change the date format, what you have to do is that, login to the database and activate the developer mode from the settings and navigate to Settings -> Translation -> Languages, open the language that you are using,

In the above screen shot you can see the field named Date Format, here you can adjust the format as per need.

Legends for supported Date and Time Formats

For Reference:

1. How To Change Date and Time Format in Odoo
   

2. How To Activate Developer Mode in Odoo
   

Thanks