If you're working offline, your cities are saved locally on the old computer's hard drive. Default location is C:\Users\<User Name>\Documents\SimCity (It will have folders called Games, Movies, Pictures) Copy the SimCity folder onto a thumb drive and transfer it to your new PC in the same location. If you don't have a Documents\SimCity folder on your W10 laptop, just copy the whole thing over. It's probably only created when you save that first local city.
Be aware if you're working offline that the game only updates your local copy, there's not a copy on the server. So playing on the new computer will not update the old computer's version.