How to change list values of list menu when value of other list meneu is change?

for example i have a list menu(num1) contaning (2 & 3 & 4) values selected itam is 2.and i have another list menu(num2)contaning (400.500,600) values when list menue(num1)is selected 2 valeu.
I wish if list menu(num1) values selected change to 3 then in list menu(num2) change list values to (700.800.900.1000).

very thanks.
sorry for bad english!!