trying to change variable value with a def

[pythonbegginer]

Hi, i am a newbie in python and am trying to make a gui to control how fast an LED blinks with a variable named "delay". i am using this code

from tkinter import *
from tkinter import ttk
import RPi.GPIO as GPIO
import time

root = Tk()

redLED = 26
GPIO.setmode(GPIO.BCM)
GPIO.setup(redLED, GPIO.OUT)


global delay
delay = .40
while True:
   GPIO.output(redLED, GPIO.HIGH)
   time.sleep(delay)
   GPIO.output(redLED, GPIO.LOW)
   time.sleep(delay)
   def Decrease_Time():
      delay -= .05

   def Increase_Time():
      delay += .05

   button_increse = ttk.Button(root, text = 'Increase', command = Increase_Time).pack()
   button_decrease = ttk.Button(root, text = 'Decrease', command = Decrease_Time).pack()

   root.mainloop()

but i get this error when i press the Increase or decrease buttons

Error:
UnboundLocalError: local variable 'delay' referenced before assignment

any help appreciated