Kralyerg made a "Terrain Tools" mod to go with CC but it should work on its own.
It's a pretty limited thing, you can only raise or lower single squares by a certain level, you can't make smooth slopes at all, it'll always look like awkward little steps.
It's found on the CC site in the download section (which requires registration on the site) here: https://www.blackliquidsoftware.com/forum/Thread-CC-Terrain-Tools

I really recommend saving on a different file before / after you start using that tool, so you always have a clean pretty town to go back to if you mess up your terraforming and it looks too ugly, because once you've changed the elevation of a square, you can never really get it back to how it was before. Terrain color can look weird too.