firmware v1.20 support DSi 1.45 and 3DS 6.0.0-11x 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.