import sys, math, random
from PyQt5 import QtCore, QtGui, QtWidgets
from app import Ui_MainWindow

app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)

MainWindow.show()

x=random.randint(1, 6)
y=math.factorial(x)
ui.doneres.setText(f'Число: {y}')

def gen():
    x=random.randint(1, 6)
    y=math.factorial(x)
    ui.doneres.setText(f'Число: {y}')

ui.again.clicked.connect( gen )

sys.exit(app.exec_())