If you want to edit lightmaps by hand you should edit their HDRI versions: lightmapN.exr, and then run post-processing. Please find more information this thread: Edit lightmap externally
However, to make the wall tops brighter the best solution is to:
- Extract the wall tops to a new object in the 3D modelling tool.
- In Shapespark, before baking, assign this new object Custom lightmap resolution of 0 in the Objects tab.
Furthermore, if you assign the wall tops a dedicated material, you will be able to easily adjust the color of the wall tops after the scene has been baked.