Take it back to the mechanic and get him to make good his poor work.
If you do the work yourself be carefull, the cars weight must be on the wheels or you risk injury if the spring makes a bid for freedom.
Remove the plastic cap and you will see a nut fastening the top plate, remove this nut....there is a hex or torx recess in the end of the damper rod to stop it rotating as you undo the nut.
With the plate off you should see another nut underneath which should be tightened fully, do not remove this nut unless you fit spring compressors first. Tighten the nut fully, the nut clamps the top support bearing against a shoulder on the damper rod, use the hex/torx in the end of the damper rod again to stop it rotating.
Im guessing the mechanic loosened the top nut to make it easier to move the strut around when trying to separate the bottom ball joint (or even removed the whole strut and wishbone as one assembly).....but I dont understand why he would have touched the lower nut.
The top plate is trapped between the two nuts so if the bottom one is loose (too far up the threads of the damper rod) the top plate will appear to be too high, just like your picture.