If you still have your original modbuilder source for the map, just revise it, rebuild, and upload again. It will replace the original with the updated one.

Adding an image: This guide tells how to edit the xml file -
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=397965314

You could probably do it in the Mod Builder instead.

Either way, you have to revert to the 32-bit version of the game or your image will not upload right.