is the spair valve a stick on (peel off sticker then place on the bladder) or is it a glue on (sandpaper glue allsorts of mess)
the stick ons quite easy you just cut out as close as yu can the old valve then take a deep breath scrunge up your face and place the replacement sticky valve onto the whole on the bladder. if for some reason the sticky patch is smaller then the hole in the bladder you can cut up an old bladder and insert it into the whole after youve cut a circle for the valve to go in.

pretty bloody impossible to understand this in text so im sure you can go to youtube and look it up.

Not sure about the newer Naishs but more & more brands are changing to welded valves on latest models.
You wont get these off in boiling water - the only way to tell is to try.
If the boiling water treatment doesnt do the trick then its welded. You have to cut out the old valve.
Only successful way Ive found is to use a scalpel (available at any chemist).
Cut around the base of the valve leaving the whole flange of the old valve in place.
Do the normal clean-up with the alcohol swab then apply the stick-on valve over the old valve flange.
If you think you may have stuffed it you can always re-inforce it with an oversized patch of Tear-Aid (centre hole cut out & corners rounded) over the new valve area - sealing well out to the original bladder.

If you havent been careful enough you may find air will still leak out at the base of the valve stem.
Time for some professional help then.

HINT : Make sure the valve cap is angled correctly to line up as per the original fitting.
Otherwise your lockdown tab may not allow for the cap to be fitted correctly.
(easy mistake - only obvious when its too late. Another of those disappointing "DOH" moments you never do twice.)