how to update real time data on tkinter label?

I am showing encoder value on tkinter label at the same time I have a function which is updated in every 100ms. So my problem is my encoder value is not updated in real time. How to solve this issue ? My code is here

if __name__ == "__main__":
   import os
   import board
   import busio
   import pigpio
   import serial
   import tkinter as tk
   import rotary_encoder
   import Adafruit_MCP4725
   import RPi.GPIO as GPIO
   from PIL import ImageTk,Image

   s=serial.Serial(port='/dev/ttyS0',
                baudrate=9600,
                parity=serial.PARITY_NONE,
                stopbits=serial.STOPBITS_ONE,
                bytesize=serial.EIGHTBITS,
                timeout=0.05)


   dac=Adafruit_MCP4725.MCP4725(address=0x61)
 
   pi = pigpio.pi()
   path="/home/pi/logo.png"
   root=tk.Tk()
   img=ImageTk.PhotoImage(Image.open(path))
   panel=tk.Label(root,image=img)
   panel.pack()
   panel.place(x=930,y=1,height=50,width=70)
   root.title("Dynalec")
   root.iconphoto(False,img)
   root.geometry("1000x600")

   Dsel = 0
   cal_c= 0

   pos=0

   rpm = 0
   torq=0
   arr = [0,0,0,0,0]


   def callback(way):  # interrupt event sense on pin no 17,18

        global pos

        pos += way
        if pos >= 9999:
          pos=9999
        if pos <= 0:
          pos=0

        var.set(pos)
        dac.set_voltage(int(pos/2.441))

    def rpm_update():
    global rpm
    s.write(str.encode(i))
    time.sleep(0.1)
    if s.inWaiting():
       rpm = s.readline(s.inWaiting())
       rpm = rpm.decode('utf-8',errors='ignore')
       rpm = rpm[5:-1]
    var2.set(rpm)
    
   def update():
       global rpm,torq

       try:
         rpm_update()
         var3.set(torq)

         pow = (float(torq)*float(avg_rpm))/9549
         pow=format(pow,'.2f')
         var4.set(pow)

       except ZeroDivisionError:
         pass

       root.after(100,update)

   #RPM Label
   L1=tk.Label(root,text="RPM/ %TC posn",fg="blue",font=("TimesNewroman",30,'bold'))
   L1.pack()
   L1.place(x=65,y=380)

   #RPM DEMAND value

   var=tk.StringVar()
   L2=tk.Label(root,fg="red",textvariable=var,font=("TimesNewroman",35))
   L2.pack()
   L2.place(x=65,y=431,height=50,width=280)

   #PRESENT RPM Label
   L5=tk.Label(root,text="RPM",fg="red",font=("TimesNewroman",45,'bold'))
   L5.pack()
   L5.place(x=220,y=55,height=70,width=240)
   #PRESENT RPM value
   var2=tk.StringVar()
   L6=tk.Label(root,fg="black",textvariable=var2,font=("TimesNewroman",70,'bold'))
   L6.pack()
   L6.place(x=220,y=116,height=90,width=240)
  
   """
   torque and power label for `var3` and `var4`.
   """
   
   

   decoder=rotary_encoder.decoder(pi, 17, 18, callback)
   root.update()
   root.mainloop()

I think due to root.update() this issue arises. But I need root.update() function also for this project. So I need to update var.set() value when ever data will be there but var2.set() in every 100ms. Do I need multithread?? So how can I ressolve this .