With the original cart
Assuming you haven't done any of that if you want to run and update emuNAND
1- launch the gateway boot screen ( while holding L )
2- backup system NAND will take about 10 minutes after that move the NAND from 3DS SD card to your PC and keep it safe.
3- from the same gateway boot screen select format emuNAND ( will take 5-10 minutes )
4- boot gateway mode
5- open settings to make sure that everything is ok
and you must see GW before the firmware version that means you're on emuNAND now
6-
don't exit or close the settings ( or you'll revert back to the original NAND ) open internet settings and make sure the WiFi is on and set up your internet connection
7- after setting up your WiFi agree to the terms of service (
don't close the settings like I said make sure there is GW before the firmware version) open other settings and system update
8- download the update
9- after the update the 3DS will restart after that open settings and make sure your original firmware is still the same 4.x or lower
10- if you succeeded in everything when you open the the classic mode or gateway mode check your emuNAND it should be 7.x
Done
FYI: when you close the settings the 3DS reverts back to the original firmware so you'll have to boot the gateway again.
Also here's a guide might be outdated but it could help you
http://gbatemp.net/threads/gw3ds-2-0b-how-to-succesfully-update-your-emunand-w-pics.358557/