Well, you need to edit the info.json file. If you haven't got one, you can take one from the official maps ([...]\Steam\steamapps\common\BeamNG.drive\content\levels\[an official level]\levels\[an official level]\info.json) and put it in your map directory. Then just open it with a text editor. Like that you should be able to modify the data in it, including the name of the level and of the preview image.
