как вытащить значение переменной из класса и переместить в другой класс?

Я отметил для вас строки, в которые надо внести изменения. Если что-то не понятно - спросите.

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)
        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", "посмотреть путь к фалу"))
        self.label.setText(_translate("MainWindow", "TextLabel"))


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.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(60, 40, 101, 23))
        self.pushButton.setObjectName("pushButton")
        self.pushButton_2 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_2.setGeometry(QtCore.QRect(220, 40, 151, 23))
        self.pushButton_2.setObjectName("pushButton_2")
        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.setText(_translate("MainWindow", "выбрать файл"))
        self.pushButton_2.setText(_translate("MainWindow", "посмотреть путь к файлу"))
        self.label.setText(_translate("MainWindow", "TextLabel"))
        self.pushButton_3.setText(_translate("MainWindow", "следующие окно"))


class MainWindow1(QMainWindow, Ui_MainWindow):              
    def __init__(self, parent):                                                  # +++ parent 
        super().__init__()
        self.setupUi(self)

        self.parent = parent                                                     # +++ parent     

        self.pushButton.clicked.connect(self.conclusion)

    def conclusion(self):
#        self.label.setText(self.name)
        self.label.setText(self.parent.name)                                     # +++ 


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

        self.name = ''                                                            # +++ 
        
        self.pushButton.clicked.connect(self.open_file)
        self.pushButton_2.clicked.connect(self.conclusion)
        self.pushButton_3.clicked.connect(self.MainWindow1)

        self.MainWindow1 = MainWindow1(self)                                      # +++ self

    def open_file(self):
        self.name, _ = QtWidgets.QFileDialog.getOpenFileName(self, 'Выберите файл', '.',  
            "Файлы Exсel (*.xlsx)") 

    def conclusion(self):
        self.label.setText(self.name)

    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_())