Is there a simple way to lower 0.5 square meters of concrete by 2mm?
The easiest way is with an angle grinder and a concrete surfacing disk, these use carbide blocks or diamond impregnated ones that grind the surface.
2mm is not a huge amount but enough at 1/2 meter square that doing it by hand will take a really long time, using a surfacing disk is cleaner and easier than masonry grinding disks
Amazon has one similar to what I have used on small jobs (4-1/2” angle grinder)
Search for grinding wheels for concrete and masonry the more segments the disk has the better the finish and longer it will last.
After surfacing the surface is usually much smoother and looks different I have used a strong muriatic acid wash to “rough up” the surface.