If I remember correctly, you already had B9S and Luma installed. Reading Nintendo's stupid threats before the update starts, really gives a bad feeling. But put these feelings away. The cfw prevents itself from being overwritten and nothing bad will happen.
View attachment 149499
One addition to this very short answer from the guide:
The last update 11.8 broke Luma compatibility (Nintendo did this probably
not on purpose) and lead to the infamous "Failed to apply 1 Firm patch(es)" message. Yesterday there was yet another
cry for help (not a big deal, only downloading new Luma). If there's ever a version 11.9 or 12, wait some days before updating and read on GBAtemp. If nobody reports problems, you can safely update. There will be probably a thread about any update.
Updating is safe for
you now, because you have the latest version of Luma if you followed the guide just some days ago.