Nintendo has come out with a new firmware, 6.2.0-12U, which adds additional street pass functionality, and blocks flashcarts. So, best hold off on updating until your flashcart has an update released.
firmware v1.21 support DSi 1.45 and 3DS 6.2.0-12x Download from here. Important: Please make sure the power is enough! And don't do anything unnecessary!
Q:How to upgrade DSTWO firmware? Step 1: Download the firmware.. Step 2: Unzip and copy the upgrade file such as "dstwoupdate.dat" to MicroSD. Step 3: Make sure you have DSTWO EOS system in your MicroSD, if you don't have, please download: DSTWO EOS v1.11(2012-12-12). Step 4: Find a DS console (NDS\DS\DSLite\DSi\3DS) which can run this EOS system, if you have the latest version console can not run EOS system, sorry, you have to find one. Step 5: Turn on the power, EOS will detect the upgrade file, press A to upgrade.
Now, the DSTWO support the latest version console. Off cause if the firmware declared support the latest version console.
There's one near where I live, in a local shopping mall. I love my 3DS, but the Nintendo Zone only really pushes Pokemon, which doesn't really interest me(my FAVORITE games for the DS series of consoles are Professor Layton and Etrian Oddysey.