Handlebar re-alignment

If you mean that the stem is point at 90 degrees from the from wheel, then you need to loosen the stem bolts, adjust the stem and handlebars so they are pointing the correct direction and tighten the stem bolts.

There will be most likely two stem bolts on your stem. In the picture below, the top bolt is the top cap bolt and the bottom bolts are the stem bolts. Usually, stem bolts are pointing from two different directions, as in the picture, but not always.

Do not over tighten any of the bolts. There should be instuctions on how tight they should be either on the stem or in an instuctions manual.

Also, you can see this video

As abdnChap says rotating the stem around the steerer tube can be accomplished by loosening the stem bolts.

However, you may need to re-adjust the headset bearing pre-load if you loosen the stem bolts, and the pre-load may not have been set properly when the bike was assembled.

To pre-load the bearings the bolt in the top-cap is tightened to pull the fork upwards slightly before the stem bolts are fully tightened. There should be no play of the fork or bars in the head tube, but the bars should turn freely. You can check for play by applying the front brake, holding the bars and rocking the bike backwards and forwards.

This Park Tool video shows how to install a stem and includes alignment and setting bearing pre-load. The pre-load adjustment procedure starts at about this point.