By hitting the 'Open Editor' hotkey (default shift+e) on any map you can enable edit mode. Note that this disables your ability to submit scores without restarting or loading a previous save.
With edit mode enabled, you can access the editor with a new button in the top right UI. Clicking this will open or close the editor window on the right side of your screen. This is where you can modify almost anything about a map, from the placement of units to the properties of the different terrain types.
The editor has 8 tabs; the details of each tab can be found on the map editor pages.
This video was designed for mapmaking, not the editor in general but it covers most of the basic uses of the editor.