Copy paste from an old thread
TL;DR: DOWNGRADING FROM ALL FIRMWARES UP TO AND INCLUDING 10.6 IS NOW POSSIBLE
https://gbatemp.net/threads/working-downgrade-from-10-6-
to-9-2.418037/
>What the fuck is happening? English, please.
Memechunkhax2 is an exploit used to downgrade 3DS firmware. Most importantly, it can downgrade to 9.0-9.2, which are ideal firmwares that can be further exploited for access to CFW, and thus warez and faster homebrew and emulators via kernel mode. This is possible on any firmware up to and including 10.3. Memchunkhax2 requires access to homebrew in order to run.
Memchunkhax3 is an exploit, much like it's predecessor, to downgrade 3DS firmware. This time, it works on 10.4-10.6, and is considerably more stable(?). As it is a new exploit that just popped out of nowhere, it may be untested and unstable for some systems and firmwares. Use this exploit at your own risk.
>How do I get homebrew and prepare for CFW?
First, pour yourself a large glass of just fuck my shit up fam and then find your firmware version below. Then see http://smealum.github.io/3ds/, unzip the starter kit to your SD card, and follow instructions to setup the Homebrew Launcher. Note that Tubehax is dead on all devices, all firmwares. Ignore it.
<9.0:
>You'll want to update to 9.0-9.2, preferably 9.2. There are ways to update straight to 9.X with CFW, or you can find a game card that has a 9.0-9.2 update on-cart and use that. Lurk GBAtemp for details.
9.0-9.2:
>You have no use for memchunkhax2/3. Install a CFW of your choice.
9.3-9.8:
>Use browserhax, optionally install menuhax as a more permanent solution. Downgrade with memchunkhax2.
O3DS 9.9-10.3:
>If you have the update nag that prevents you from using the web browser, change your system's date to January 1st, 2000 at 00:00, initialize savedata, and immediately use browserhax. Don't exit the browser using the home button, or you'll have to reinitalize save data.
N3DS 9.9-10.1:
>If you have the nag, you can beat it by opening the web browser as fast as you can from boot before the update nag kicks you out. Bookmark the applicable browserhax link, use as quickly as possible. You'll have about 30 seconds. Alternatively, you can use Smashhax if possible. Install menuhax immediately afterwards. Use memchunkhax2.
N3DS 10.2-10.3:
>As of writing, from these firmwares upwards on n3DS systems ONLY, you should not have the browser nag. Use browserhax as normal, install menuhax, use memchunkhax2.
10.4:
>Browserhax and menuhax are usable. Ironhax does not work on this firmware as 10.4 REQUIRES you to update the game before launching it. Use memchunkhax3.
10.5:
>Alongside this update came the inability to play online unless you're on the latest firmware. All 10.4-compatible 'sploits are usable. Use memchunkhax3.
10.6:
>Fixed browserhax and menuhax (again). This, as the most recent FW, is needed to use online services. You can safely update emuNAND to this FW. Use a card-based exploit like oot3dhax for now, then use memchunkhax3.
>CFW??!?¿?
rxtools is one of the few CFWs being actively developed, and it works on all models, but you have many other options too. Usually, you'll want a CIA installer and hblauncher_loader/menuhax installed to your home menu no matter what you do.
Currently, ReiNAND and CakesFW are the only CFWs supporting n3DS 9.6+ emuNAND. Rxtools 3.0 will feature n9.6+ emuNAND when it is released.
Do further, thorough research on your own. Ask smart questions here.
>What's a .CIA?
See: https://gbatemp.net/threads/what-is-a-legit-cia-and-other-noob-cia-questions-answered.386705/
>What's emu/sysNAND?
See: https://gbatemp.net/threads/guide-the-emunand-save-data-mega-guide-gbatemp-doesnt-deserve.378491/
>I set up emuNAND! But where is FBI/[CIA Installer of Choice]?
If it isn't showing on your home menu with CFW on, your earlier install of it on sysNAND didn't work. Inject it into the H&S app: https://gbatemp.net/threads/release-inject-any-app-into-health-safety-o3ds-n3ds-cfw-only.402236/
>ROMS WHERE
3DSISO, mega3dsroms, /scurv/, private trackers, usenet, or Google.
We also have /scurv/'s volafile (https://volafile.io/r/acH6c7), where you can download and upload game .cia's if you wish. Some downloads corrupt, so vola is much better as a quick tech support chat.