As long as you are logged into Facebook on your old device, it will be extremely easy.
On your new device:
- Install the SimCity BuildIt game via Google Play or App Store depending on your system (Android or iOS, respectively).
- Once installed, open the game, and go through the short tutorial on startup.
- Once done (it usually takes just a minute), an icon called Settings (symbolized by a screwdriver and wrench crossed) will be enabled on the top right, where you have to click.
- Log in to Facebook, and enter your credentials (username and password of the account associated with your old city in this social network) when prompted.
- It will return you to the game and ask you which city you want to upload.
- Carefully select the city you had on your other device. It will warn you that the city you have created with this new device will be permanently deleted.
(*) Contributed by Atlantis