Welcome to the site!
I can't help with your sight problem except with some general advice, as I've never personally tried to move an XD sight.
First, make sure if the sight has a locking screw, you loosen it. Usually, if there IS a lock screw, it will be visible from the top of the sight, passing through from top to bottom. You will need a very small hex-head wrench to loosen it (it's probably best to remove it all the way and put it in a safe place so it doesn't get lost).
Second, to get the sight to move by tapping it, you have to hold the slide immobile so the slide doesn't move (instead of the sight) when you whack it. It's best to remove the slide from the weapon, and clamp it in a well-padded vise (use thick leather or clean, very soft wood for padding). Don't over-tighten the vise or you might bend/warp the slide or the rail grooves.
Finally, if you still can't get the sight to move, consider spraying a bit of WD-40 or other penetrating oil on the edges of the sight cut in the slide, and letting it seep in for 1-2 days. Sometimes it can make all the difference in the world.
Hope this has been helpful.