Modding is a planned feature in Manic Miners. It will allow users to create their own Buildings and Vehicles.
Currently, modding is not possible apart from swapping certain specific assets, such as models, sounds and textures, by copying the exact folder structure of the game into your own project and compiling your own Unreal asset files, which can then replace the existing game assets, hopefully without crashing. Official modding support will be added some time after V0.3.
If you are looking to modify the level file and want to do so without encountering errors, visit the Level Data File page.
If you are looking to write level scripts, please visit the Scripting page.