The README.md file inside the download includes those instructions:
# mapsOrder.html
### Reorder maps list outside of RPGMaker's UI
* Drag and drop the file onto / or open it with a web browser (Chrome or Firefox)
* Drag and drop the mapInfos.json file from your project's data folder
* Click on a map you want to move
* Click the + next to the map you want to move above, below, or inside
* Click above, below, or inside on the various buttons.
* Watch it move!
* Click the save button, and replace mapInfo.json with the file it makes
