Scroll down to the area called URLs Setup, and look for the Shop domain name input field.
Here you can change the url to either use www (example: www.mydomain.com) or not (example:mydomain.com)
Now save your settings.
In order to make this work you need to let the server know about the change by updating the .htaccess file. This is done conveniently from within the Prestashop admin interface.
Then click the Generate .htaccess file button (If you can’t generate the .htaccess file from here, you will most likely be asked to change the write permissions for the file. You can do so for example via FTP).
Now you can go back to your shopfront and see if the changes took place.