QTreeWidgetItem* item = ...;
if( QTreeWidgetItem* parent = item->parent() )
{
     // item is a child (= key)
     QString keyPath = QString("%1/%2").arg(parent->text(0)).arg(item->text(0));
     QVariant val = iniSettings->value( keyPath ); // reading
     iniSettings->setValue( keyPath, val ); // writing
}
else
{
     // item is a root item (= group)
}

 QTreeWidgetItem& Widget::createRoot(const QString &groupname)
{
   QTreeWidgetItem *root=new QTreeWidgetItem(displayTreeWidget);//here parameter is QTreeWidget
   root->setText(0,groupname);
  displayTreeWidget->addTopLevelItem(root);
return *root;
}

//method to create children and attach them to toplevelitem
void Widget::createChild(QTreeWidgetItem &rootaddress,const QString &key,const QVariant &value)
{
  QTreeWidgetItem *child=new QTreeWidgetItem;
 child->setText(0,key);
child->setTextAlignment(0,Qt::AlignCenter);
child->setData(1,2,value);
rootaddress.addChild(child);
}
//these two methods are called while initial group creation
//mentioned with bold text in the code below

Groups have been created initially using code:

{
        iniSettings->beginGroup(group);
        QTreeWidgetItem &rootaddress=**createRoot(group);** //it creates group text as node on QTreeWidget object.
         foreach( QString key, iniSettings->childKeys() )
         {
                QVariant val = iniSettings->value(key);
                ***createChild(rootaddress,key,val); *** /adds key and value of the group as the child nodes to the QTreeWidget using 
                rootaddress.
         }
     iniSettings->endGroup();
} 

Now if a user changes(or edits) any value on the QTreeWidgetItem on QTreeWidget object, I want update this values to existing group key/value in the QSetting and the .ini file ,

 void Widget::itemChanged(QTreeWidgetItem *item,int col)
{
QTreeWidgetItem *parent=item->parent();
qDebug()<<parent->text(0);//App is getting crashed here, do not know why? how to get text from this root?

iniSettings->beginGroup("mygroup");
QVariant val = iniSettings->value("mykey");
iniSettings->endGroup();

QVariant val = iniSettings->value("mygroup/mykey");

how to get parent node text using current QTreeWidgetItem?

QTreeWidgetItem::parent()

So why to call again QSettings::beginGroup.

because this sets the group to read/write from (if you do not specify the group in the key path already).

There are the 2 following ways to read the key mykey from the group mygroup

iniSettings->beginGroup("mygroup");
QVariant val = iniSettings->value("mykey");
iniSettings->endGroup();

QVariant val = iniSettings->value("mygroup/mykey");

to write a value into a group you need either set the key of the value to group/key

Now if a user changes(or edits) any value on the QTreeWidgetItem on QTreeWidget object, I want update this values to existing group key/value in the QSetting and the .ini file

yes, and why is my advise wrong here?!

{
        iniSettings->beginGroup(group);
        QTreeWidgetItem &rootaddress=createRoot(group); //it creates group text as node on QTreeWidget object.
         foreach( QString key, iniSettings->childKeys() )
         {
                QVariant val = iniSettings->value(key);
                createChild(rootaddress,key,val); //adds key and value of the group as the child nodes to the QTreeWidget using 
                rootaddress.
         }
     iniSettings->endGroup();
} 

Now if a user changes(or edits) any value on the QTreeWidgetItem on QTreeWidget object, I want update this values to existing group key/value in the QSetting and the .ini file ,