With the old valve out of the way you can remove the old brass sleeve and compression nut from the pipe. A compression sleeve puller could come in handy if you have trouble removing and old brass sleeve and nut.
Note: Many times the old compression nut will fit the new valve and you could then just tighten the new stop valve on with the old sleeve and nut. The fit can be tested test by putting the new nut on to the old valve and if it threads on easily by hand it is the same thread.
But it It is always a good idea to change all of the pieces, particularly when the old valve is quite old and worn.