Usual is the undo the alan key or imbus key bolt in the middle of the handlebar stem. only undo this a couple of turns until the handle bars can move from side to side if you undo too much the locking sears inside the stem may fall off into the folks and are difficult to retreived and reset back onto to your stem. and its usually a dirty job.

Once your handlebars are loose pull up to desired level and retighten.
best way is to stand with the front wheel griped between the legs facing backward between the handlebars lifting up with a left to right rocking motion on the bars. Do not over tighten as this may cause the stem to bond inside the forks and not release in years to come.
it is usual that a bit of grease is in this area to stop the metals bonding inside each other.