This editor should be pretty self-explanatory but there are some things to note.
Beware that this might introduce non-working states into your save file if used incorrectly. For example when giving yourself key items in certain situations. Selecting certain maps or spawn points may lead to a crash or falling out of bounds and dying. Spawn points usually are indexed starting from 1 with the exception of letterboxes and respawn points after falling down somewhere. While you can change the character to Nier (Gestalt) or Kainé the game loop will likely set it to another character as certain conditions are not fullfilled. The entire source code of this site and of other related stuff can be found on GitHub.
Feel free to contact me on social media if you have any questions, requests or suggestions.