There is a special tool to make a screenshot of your map. Drag BP_MapScreenshot blueprint to the world and change its Z location so that the blueprint is above all your map objects. Don’t forget to delete it after you done otherwise it’s render target will load your system.
Here you can change the resolution of your map screenshot, in RT_MapScreenshot. For better quality try to make its resolution larger.
Now make a 2D texture out of the rendered image. You can export it and change whatever you want.
Open the DT_LevelMap data table and add a new row with the name of your map. Choose your new image here.