I don't like the Birch color much but I got a couple of those to use but would rather have the darker color. How do I go about making this change without ruining the wardrobe or the pieces please?
If you're feeling adventurous you could try spraying on a tinted top coat, that way you could achieve the ebony(ish) color and preserve the grain pattern. Its a tricky technique and it must be sprayed but it can be done.
There are a few variations but in general you add 10-20% dye stain to a finish like urethane or lacquer and spray on enough coats to get the color saturation you're after. Prep the surface as described by Steven and then apply an un-tinted clear sealer coat (my personal preference is Zinser's shellac based sealer). Sand with 280 and then add two coats of your tinted top coat, spraying in different directions (left to right then up to down). If it requires more coats, sand with 320-400 and continue on, sanding every few coats. If you have to let it sit for an extended period between coats (more than a day) be sure to sand to promote adhesion.
Seeing how it isn't real wood, rather just a thin laminate on top of MDF, I wouldn't bother trying to stain it and instead would just paint it. Painting it shouldn't be any different than painting anything else really - lightly sand, prime and paint! Depending on the color and type of paint, you might need two or more coats.
If you haven't yet assembled the furniture, it would probably be easier to paint before assembling.