This is more a PHP question than Wordpress question. You can simply use [`str_replace`][1] to change the `/` in your date to `-`. You are not changing the format, so no need for `date_format` or any other date related functions

You can try something like

    $date = '03/09/2015';
    $modified_date = str_replace( '/', '-', $date );
    echo $modified_date;

This will output `03-09-2015`

 


  [1]: http://php.net/str_replace