over 4 years ago - Avanya - Direct link
To back up your saves just make a copy of the files and keep them somewhere. The save files are found here: C:\Users\\AppData\Local\Colossal Order\Cities_Skylines\Saves

As for mods, you can also copy all the mods and assets from the workshop folder, but those subscriptions are tied to your Steam account, so if you log in with the same account on another computer, it will download those files. Then you just need to enable everything.

Workshop content is stored here, though the path may vary a little depending on where your Steam is installed.

C:\Program Files (x86)\Steam\steamapps\workshop\content\255710