How to update a tkinter Label()?

I'm working on a pretty complicated program for a prototype of an app. But I am not so great with tkinter, and because I was having a couple of problems with updating Label() objects, I made a little program to try that. My code for that little program is:

from tkinter import *


def change_func():
    labeltext.set("yep done")


window = Tk()

labeltext = StringVar()
labeltext.set("change meh")
label = Label(textvariable=labeltext)
button = Button(text="change", command=change_func())
label.pack()
button.pack()

window.mainloop()

However, when I run the program, the window that comes up just says yep done above the button saying changed. I haven't clicked the button at all. I have tried many different methods that I found online to change the value, none have worked yet.

On my more complicated program, the final value of the Label() that I am changing the text of is not defined until you click a button to define it, and it shows the original value no matter what you do. Please can someone help!