Update xml file using qtXml

i'm just trying to modify my xml file using QtXml lib , so i need some help b'cause i don't understand how it's work. so this is my xml file config.xml :

<?xml version="1.0" encoding="UTF-8"?>
<BBBBOX>

    <ListeConfigurationLS>
        <ConfigurationLS Id="1" Nom="URL"   NumeroPort="8" Vitesse="57600" BitsDonnees="8" BitsParite ="0" BitsStop="1"/>
        <ConfigurationLS Id="2" Nom="BOR"   NumeroPort="4" Vitesse="57600" BitsDonnees="8" BitsParite ="0" BitsStop="1"/>
        <ConfigurationLS Id="3" Nom="XBEE"  NumeroPort="7" Vitesse="57600" BitsDonnees="8" BitsParite ="0" BitsStop="1"/>
        <ConfigurationLS Id="4" Nom="BBI"   NumeroPort="5" Vitesse="57600" BitsDonnees="8" BitsParite ="0" BitsStop="1"/>
    </ListeConfigurationLS>

</BBBBOX>

what i want to do :

Change NumeroPort = "5" to NumeroPort = "9".