title: Update StringVar in tkinter entry
body: <p>Could someone please tell me the accepted way in python 3 to update the stringvar in a tinker entry. In the code below I would like text in the entry to change from "default text" to "new text" once the button is pressed. Thank you.</p>
  
  <pre><code>import tkinter as tk
  from tkinter import *
  from tkinter import ttk
  
  def get_var():
      new_text = "new text"
      print(new_text)
  
  window = Tk()
  window.title("My Program")
  
  v = StringVar(window, value='default text')
  case_name_lbl = Label(window, text="Case Name:").pack(padx=10, pady=10, 
  side=LEFT)
  case_name_entry = Entry(window, textvariable=v)
  case_name_entry.pack(fill=tk.X, expand=True, padx=10, pady=10, side=LEFT)
  
  btn = Button(window, text='File', width=10, command=get_var)
  btn.pack(in_=window, padx=10, pady=10, side=LEFT)
  
  window.mainloop()
  </code></pre>