If you're playing, then the game itself is already downloaded.
Do you mean you want to download and play your region & cities offline? Any cities that you are playing online can be downloaded and continued offline, in Single Player mode. Step by Step instructions are here: https://answers.ea.com/t5/SimCity-2013/SimCity-update-10-3-release-notes/td-p/4084353
If you backed up your computer before you formatted the drive, copy the contents of your SimCity\Games folder from the backup to the same folder on your hard drive (default location is C:\Users\<YourUserName>\Documents\SimCity\Games). If you didn't back up before you formatted, then unfortunately there is no way to get offline files back.
If you were always playing online, it's just a matter of choosing the server your regions are on. Then your previous games will show up.
I hope this helps. Happy Building!