Open the Level Editor window “Tools → Level Editor”.
If the currently opened scene is not a LevelEditor.scene you will see a warning and button Open Level editor scene feel free to press it.
Select the Items Tab at the top of the window.
Press the Edit enum button
Click little + button at the bottom of the list. Also at this state you can deleted not needed elements.
Enter a name for the new element and click Save
Find folder Project Data/Game/Prefabs/Items it contains level elements.
Duplicate any of existing items (Ctrl + D or Cmd + D). Rename it to your liking. And double click to open it.
As you can see it has a child with visuals. Replace it with your new model. Make sure to add the Items material to every Mesh Renderer of your new model.
Now select the parent object (in this example called SpikesItem). Find Level Item Behavior script in the inspector window. Change Item field to the new value (the name you created in step 6)
Also, adjust collider located at the parent object. You can change it’s Center or Size fields, or change the collider type. In the example I already have Box Collider and it fit’s fine with a slight adjustment of size, but you can also remove it and add Capsule or Sphere collider if it would fit better the new shape.
The last step, return to the Level Editor window, tab Items, and drag your newly created prefab into the field with the item name.
