import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QPushButton

class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()

        button1 = QPushButton(self)
        button1.setText("Mehet")
        button1.move(50, 100)
        button1.clicked.connect(self.on_click_button1)

        self.setGeometry(100,100,320,200)
        self.show()
    

    def on_click_button1(self):    
        self.setWindowTitle('egy')

if __name__ == '__main__':
    app = QApplication(sys.argv)
    mainWindow = MainWindow()
    sys.exit(app.exec_())