python 3.x - change the position of already packed widget in tkinter

Question

Here is the code,

from tkinter import *
root = Tk()

update_button = Button(root, text='Update')
update_button.pack()


def button():
    frame1 = Frame(root)
    frame1.pack()

    button1 = Button(frame1, text="Button 1")
    button1.pack(side=LEFT)

    button2 = Button(frame1, text="Button 2")
    button2.pack(side=LEFT)


button()

root.mainloop()

I want button1 to be stacked upon button2 when I click update button. Help please.