how to use update in tkinter code example

Example: how to update values in tkinter

import tkinter as tk

win = tk.Tk()
max_amount = 0
label1 = None #just so it is defined

def fun():
    global max_amount, label1
    max_amount +=100
    label1.configure(text='Balance :$' + str(max_amount))

btn = tk.Button(win,text = 'Change', command = fun)
btn.grid()
t1 =str(max_amount)
label1 = tk.Label(win,text = 'Balance :$' + t1)
label1.grid()

win.mainloop()