this is a slight advanced method so I still suggest doing it Plailect's Guide's way. However if you insist here're a few steps on how to do it.
0. BACKUP first your RedNAND.
1. Follow Part 4 Section II of Plailect's Guide to inject FBI.
2. Download 3DNUS from here
https://github.com/zoltx23/3DNUS/releases
3. Run 3DNUS as Admin and download the firmware you want. (Here's an example: If you're going to download the latest USA region Firmware, enter "11.0.0-33U" without the quotation marks and click begin download.)
4. After downloading, you'll find an 11.0.0-33U folder inside the 3DNUS directory. rename it to "updates" and place it to the root of your SD card, make sure there're no other updates folder on the SD card before doing so.
5. Download the sysUpdater from here
https://github.com/profi200/sysUpdater/releases and extract the sysUpdater.cia to the root of your SD card.
6. Boot your 3DS into RedNAND and open Health&Safety which is no FBI.
7. Navigate to your SD and install SysUpdater.
8. Go back Home and you'll find the newly installed sysUpdater, open it up and press A to update.
9. That's it! If the updating was successful, your RedNAND will be on the latest firmware. If not, restore your backup and find the mistake you made before trying again.