It´s now avaible the new update for U.S 3ds version

but not with mii plaza update.
Latest System Version: 6.0.0-11U

The latest system update is here

NEW: Easy backups for your digital save data
Downloading digital games and applications from the Nintendo eShop is easy and convenient—and now, so is backing up your digital save data. With the new Nintendo 3DS system update, you can back up data to your SD card at the touch of a button—no need to transfer your card or use a PC.

If you have system update 2.0.0-2U or higher, you don’t need to do anything except agree to the update. It will be downloaded automatically when you are in range of a wireless Internet connection and SpotPass™ is enabled. If you have never performed a system update, connect your Nintendo 3DS system to the Internet, and find the System Update option in your system settings.