Как закрывать окно при открытии нового?

Я отметил для вас строку, которую надо поменять.

from PyQt5 import QtGui, QtWidgets, QtCore
from PyQt5.QtWidgets import QApplication, QMainWindow, QStackedWidget


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(334, 150)
        MainWindow.setMinimumSize(QtCore.QSize(500, 250))
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(100, 20, 141, 23))
        self.pushButton.setObjectName("pushButton")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(110, 60, 111, 31))
        self.label.setObjectName("label")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 334, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "Cледующие окно"))



class Ui_MainWindow1(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(426, 216)
        MainWindow.setMinimumSize(QtCore.QSize(426, 216))
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(60, 90, 141, 51))
        self.label.setObjectName("label")
        self.pushButton_3 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_3.setGeometry(QtCore.QRect(270, 110, 101, 23))
        self.pushButton_3.setObjectName("pushButton_3")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 426, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton_3.setText(_translate("MainWindow", "следующие окно"))


class Ui_MainWindow2(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(426, 216)
        MainWindow.setMinimumSize(QtCore.QSize(426, 216))
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(60, 90, 141, 51))
        self.label.setObjectName("label")
        self.pushButton_3 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_3.setGeometry(QtCore.QRect(270, 110, 101, 23))
        self.pushButton_3.setObjectName("pushButton_3")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 426, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton_3.setText(_translate("MainWindow", "конец"))


class MainWindow2(QMainWindow, Ui_MainWindow2):              
    def __init__(self, parent):                                                  
        super().__init__()
        self.setupUi(self)
#        self.parent = parent                                                  


class MainWindow1(QMainWindow, Ui_MainWindow):              
    def __init__(self, parent=None):                                                 
        super().__init__()
        self.setupUi(self)
# ?        self.parent = parent                                                  
         
        self.pushButton.clicked.connect(self.MainWindow2)
        self.MainWindow2 = MainWindow2(self) 

    def MainWindow2(self):
        self.MainWindow2.show()
#        windows.hide()                                                   # ---                                
        self.hide()                                                       # +++
        

class MainWindow(QMainWindow, Ui_MainWindow1):                          
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setupUi(self) 

        self.pushButton_3.clicked.connect(self.MainWindow1)
        self.MainWindow1 = MainWindow1(self)                                     

    def MainWindow1(self):
        self.MainWindow1.show()
        windows.hide()


if __name__ == '__main__':    
    import sys
    app = QApplication(sys.argv)
    
    startmenu_window = MainWindow()
    windows = QStackedWidget()
    windows.addWidget(startmenu_window)  
    
    windows.show()
    sys.exit(app.exec_())