Assignment directive not assigning new value to a symbol in emu8086

Why doesn't the following code assign new value to a symbol X using Assignment Directive ( = ) in emu8086:

.model small
.data

        X = 8

.code
.startup

       mov ax, @data
       mov ds, ax

       mov bx, X

       X = 6      

       mov bx, X 

       mov ah, 02h
       mov dx, bx   
       add dx, 48
       int 21h     ; It should display 6 but instead it display 8. 

       mov ah, 04ch
       int 21h

end