First things first: people are arguing over whether this is actually a map editor, but that’s what the creator—Guadmaz—has called it. From what I can tell, this mod doesn’t allow you to change the layout of the world in GTA V but does let you spawn objects like ramps and vehicles into that world.
The creator compares it to the Multi Theft Auto: San Andreas map editor, so if you’ve used that you should have some idea of how this will work. Here’s the description:
For the average user, this is probably of no use or interest to you so keep that in mind before you read any further!!
Other info
The type of an object can be determined by the color of the label. For buildings/ static objects, it will be red. For props it is gold. This can be changed in the config file.
As always, let me know of any bugs/ problems, and offer suggestions for how the mod can be improved! Thanks 
Controls for camera
Move – W, A, S, D
Select/ Enter Editor – Left mouse
Toggle console – T
Exit –Right mouse
Speed boost – Left shift
Controls for editor
Move Object – W, A, S, D
Set move speed – Scroll wheel
Toggle console – T
Exit – Right mouse
List of usable commands
“?” or “help” – Gets a list of available commands
“reloadconfig” – Reloads all values from the config file
“showtextures” – Prints all nested texture names for the selected object
“dumpobjects” – Dumps nearby objects to nearbyobjects.txt
Default activation key is ‘Y’, but you are free to change this in the config file
Additional Requirements
Readmore: World Improvement 1.9 for GTA 5 [Map Editor] – Mod map gta 5 pc