Copy the OTPHelper folder from the OTPHelper zip, the TinyFormat folder from the TinyFormat zip, and the Decrypt9WIP folder from the Decrypt9 zip to the /3ds/ folder on your SD card
Copy sysUpdater.cia from the sysUpdater zip, hblauncher_loader.cia from the hblauncher_loader zip, and arm11.bin and arm9.bin from the OTPHelper zip to the root of your SD card
Copy the 3ds folder from the FBI zip to the root of your SD card
From SysNAND, get into the Homebrew Launcher through the entrypoint of your choice
Open OTPHelper
Go to "NAND Backup & Restore"
Backup SysNAND and EmuNAND to sysNAND.bin and emuNAND.bin respectively
Press Select on the main menu to eject your SD card
Put your SD card in your computer, then copy over sysNAND.bin, emuNAND.bin, and the Nintendo 3DS folder to a safe folder on your computer, you will need them later
Delete emuNAND.bin from your SD card; do not delete sysNAND.bin
Make sure your WiFi is on (it has not been disabled from the SysNAND home menu settings), you will not be able to toggle it in 2.1
Steps 13 through 21 are not required if you followed Part 3 and have already injected FBI into Health and Safety
Get into the Homebrew Launcher on SysNAND through the entrypoint of your choice
Open Decrypt9, then go to "EmuNAND File Options", then select the "Dump Health & Safety" option
Press Select to eject your SD card, then put it in your computer
Extract Universal Inject Generater, then copy both hs.app from your SD card and FBI.cia from the FBI zip to the input folder
Double click go.bat
Copy FBI_inject_with_banner.app to the root of your SD card and reinsert your SD card into your 3DS
Press B on Decrypt9, then go to "EmuNAND File Options" and select the "Inject Health & Safety" option
Press down once to select FBI_inject_with_banner.app, then press A to inject
Press start, then boot your 3DS into EmuNAND using any CFW (if you followed Part 3 of this guide, you can do this by launching AuReiNAND from the homebrew launcher)
Launch the Health and Safety Application (which is now FBI) on EmuNAND
Navigate to hblauncher_loader.cia and press A to install
Launch the Homebrew Menu using the home menu icon
Open TinyFormat
Press Y to format EmuNAND
Reboot back into EmuNAND and complete initial setup without signing into your Nintendo Network ID
Repeat steps 21 through 26, this is not optional (in other words, you must TinyFormat your EmuNAND twice - if you do not, you will most likely brick your device when you downgrade)
Remove any TWL modifications done to the device or sysUpdater will throw an error (most users will not have to worry about this)
Update your EmuNAND to the latest version using system settings if it is not already
Open the eShop at least once to allow it to initialize databases
Launch the Health and Safety Application (which is now FBI)
Navigate to sysUpdater.cia and press A to install
Section II - Downgrading
Reboot into SysNAND, then get into the Homebrew Launcher through the entrypoint of your choice
Open OTPHelper, then go to "Nand Backup and Restore", then select the "EmuNAND Backup" option
Backup your emuNAND to emuNAND.bin
Press Select on the main menu to eject your SD card
Put your SD card in your computer, then rename emuNAND.bin on the root of your SD card to emuNAND_formatted.bin
Copy over emuNAND_formatted.bin to a safe folder on your computer
Delete any existing updates folder from your SD card root that may be leftover from a previous downgrade or update
Copy the updates folder from the 2.1.0 firmware zip to the root of your SD card
Reinsert your SD card into your 3DS then press Start and reboot into emuNAND
Open sysUpdater on emuNAND
Press Y to downgrade emuNAND to 2.1.0
If you encounter an error at any point during the downgrade, restore your emuNAND backup from emuNAND_formatted.bin using OTPHelper through the Homebrew Menu on SysNAND. Afterwards, you can retry the downgrade on EmuNAND, restoring from backup whenever it fails until it goes through successfully. This downgrade process can take many many tries in some situations, just keep trying until it works
If, after many tries, you repeatedly fail on 0004003000008F02.cia, you can try following the instructions here
EmuNAND will be bricked by the downgrade and you will no longer be able to get into it (a black screen when you try to load EmuNAND is normal)
Reboot into SysNAND, then get into the Homebrew Launcher through the entrypoint of your choice
Launch Decrypt9, then backup EmuNAND to emuNAND.bin
Flash the 2.1 NAND to your SysNAND by restoring SysNAND from emuNAND.bin
Cross your fingers
Reboot
If you get a black screen, boot with the SD card removed then reinsert
Section III - Getting the OTP
Go to http://dukesrg.github.io/2xrsa.html?arm11.bin on your 3ds
OTPHelper will launch, then select the "Dump otp.bin (0x108) (< 3.0)" option
Press Select on the main menu to eject your SD card
Put your SD card in your computer, then rename otp.bin to otp0x108.bin
Copy over otp0x108.bin to a safe folder on your computer.
Reinsert your SD card into your 3DS then press B to return to the main menu
Select the "Dump otp.bin (0x100) (< 3.0)" option
Press Select on the main menu to eject your SD card
Put your SD card in your computer, then copy over otp.bin to a safe folder on your computer
Ensure that otp.bin is 256 bytes and otp0x108.bin is 264 bytes
Backup otp.bin and otp0x108.bin to multiple locations (such as online file storage)
Section IV - Restoring the System
Rename emuNAND.bin on the root of your SD card to emuNAND_2-1-0.bin and copy it to a safe folder on your computer so you can downgrade back to 2.1.0 easily if you ever need to. You can delete emuNAND_2-1-0.bin from your SD card.
Copy the emuNAND.bin you created at the beginning of Section I to the root of your SD card
Delete the folders from the inside of the Nintendo 3DS folder on your SD card that are not in the Nintendo 3DS folder you copied to your computer in Section I (the Nintendo 3DS folder on your SD card should now look the same as the one from Section I)
Reinsert your SD card into your 3DS then press B to return to the main menu
Go to "NAND Backup & Restore"
Restore SysNAND and EmuNAND from sysNAND.bin and emuNAND.bin respectively