This type of stem has a wedge on the bottom which fits into the fork steerer tube. this is covered with a plastic cap

If you need to adjust the angle of your handlebar, you can do this by loosening the long nut 

clamping nut. that passes through to the wedge The handlebars will then rotate freely in the fork. Adjust to desired 

angle, and ensuring the bar is still centred in the stem, tighten the central bolt that passes through the stem to the wedge

To adjust or install this type of stem, you need to loosen the centre bolt enough so that the stem will fit in  

the steerer tube, by turning it anti-clockwise with an Allen key.

You can then insert or adjust the stem. You can rotate it left to right and you can also adjust the height. It is very important

that you observe the minimum insertion mark found on the stem and that you insert the stem beyond this. This must not be visible once the stem is in fork

Once your stem is in place and in line with the forks, making sure brake is facing forward tighten it up by turning the centre bolt clockwise

 it may be easier to readjust this with the front wheel fitted, so the stem is in line with the wheel