I'm wondering where it locates or how to open this editor during game. Or there is a mod for that?
As of patch 1.4, released march 2016, there are now mild terraforming tools in the game, no mods or dlc required.
To open them you click the "Landscaping" icon, and the select the terraforming tab. There are a couple of options like raise/lower terrain, level terrain, shift terrain.
You can load up your existing city and use a new toolbar to raise and lower the land, paint hills, cliffs, mountains, and valleys. Once you subscribe to the mod, you'll find a new icon on your dashboard, next to your budget and policies icons. Clicking it brings up a panel of terra forming options, allowing you to right or left click on the ground and raise or lower the sections of the map you've highlighted. Pressing + or - on your keypad increases and decreases the size of your brush. This way you can make small adjustments like hills, or make the brush size massive and start growing mountains.
This is what I found on http://www.pcgamer.com/in-game-terraforming-modded-into-cities-skylines/