You will then need to tighten the 5mm bolt, not too tight, too tight and you will crush the bearings and the steering will be very heavy, not enough and you will have play in the bearings, so try and remember how tight they were when you undid the top bolt.
After that then tighten to the 2 4mm bolts on the side of the stem.
Re: How do I adjust the handle bar height on a B'TWIN 540 MTB?
[ Edited ]
There's a small rubber cap which can be easily removed with a finger nail - revealing the 5mm bolt mentioned above... 👍 I've taken a couple of pictures and added them to a public album on my account should, click if you think it'll help :-)