class MainWindow : public QMainWindow
{
Q_OBJECT
QTimer *timer; // NEW
public:
void readFile(); // NEW to read the file and set to label
....
private slots:
void TimerSlot(); // NEW slot
private:
    Ui::MainWindow *ui;
};

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    timer = new QTimer(this); // create it
    connect(timer, &QTimer::timeout, this, &MainWindow::TimerSlot ); // connect it
}

void MainWindow::readFile()
{
    // QFile readALL() etc and set to ui->label
}

void MainWindow::on_pushButton_released()
{
readFile(); // read file
timer->start(120000); // 2 mins timer
}

void MainWindow::TimerSlot()
{
    readFile(); // read file again
}

while(1==1){
-> i call on_butttom_clicked
-> i define the timer in 2 minuts

} //kind of infinite while

?

I have some doubts about where or when should i use qtimer. I mean, first i clicked the button, so the information appear in the label, then i have to use qtimer to update the text automatically without clicking the buttom?

If you later wish to be "clever", you might use https://doc.qt.io/qt-5/qfilesystemwatcher.html to watch the file for changes to its content and only re-read at that point, rather than every two minutes regardless.