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ATTENTION: DOWNGRADE MAKES YOUR GAME SAVES THAT WERE PLAYED IN SYSNAND UNUSABLE. There is no way to recover them, because starting from version 6.2 another type of encryption was being used, and therefore they can not be read by older versions.
WARNING: THIS TUTORIAL IS INTENDED TO WHO ALREADY HAS A EMUNAND, AND HAS THE MINIMUM KNOWLEDGE ABOUT SYSNAND / EMUNAND / GATEWAY.

1-Go to the website (http://www.gateway-3ds.com/downloads/) and download the Gateway 3.0.1, copy the Launcher.dat to the root of your SD card (which goes inside the 3DS)

2-Go to the website (http://www.gateway-3ds.com/downloads/) and download the downgrade file THAT MATCHES THE REGION OF YOUR 3DS.If American, USA; European, EUR, etc. Put this file "gateway.dg" in the root of your SD card (which goes inside the 3DS)

3-Turn on your 3DS, enter your browser, go to http://go.gateway-3ds.com and press "L" when the low turn white screen. Now, on the GateWay menu, go to "System Backup Nand". After that, click "Boot Classic Mode". After the 3DS enter sysNAND, turn it off, place the internal SD on the computer and make a backup of it, save it somewhere safe.

4-Turn your console to enter the sysnand it, remove the MicroSD / internal SD card and your Gateway .Format the system from the conf.meu. It mwill ask you to connect to the internet to be able format, go ahead, this is normal, the sysNAND will NOT be updated, only formatted. (ATTENTION, THIS PROCESS deletes ALL YOU HAVE IN SYSNAND). Place the two back then restarts connect your console. This should 'unlink' the sysNAND of emuNAND, which is good, and also solves the bug to stay having to unwrap the .cia every time you enter the emuNAND.

5- Now in sysNAND, do the browser exploit again, and when you are in the gateway menu, go to "console downgrade". Make sure the gateway.dg file for your region is in SD root. Typically, the "Downgrade console" would ask you to back up the sysNAND, but as you did it already, you will have no problem.

6- After being full downgrade, try to click in "Boot GateWay mode". This should take you emuNAND. After that, between the settings, check whether you really are in emuNAND. If you simply close the settings menu, it will return to 3ds sysnand. After returning, enter the menu settings and voila, you're in 4.2.