There are a few YouTube videos where the fans are replaced, but all of them just have some short B-roll and voiceover, not a detailed guide.
It’s honestly not that hard though. The three front fans are screwed to the faceplate, so to access those you can undo the seven screws on the front of the unit. The fans mid-chassis are screwed to the piece of metal that runs across the case behind the backplane and drives and that is secured in with two screws on each side of the outside of the case. The tails for the fans go back to some short leads from the power distribution board on the right wall of the case, so unplug them there.
How you proceed then depends on which “mod” you want to do.
a) You can plug resistor cables between the PDB and the existing CG fans to lower the constant RPM and airflow and call it a day
b) If your mobo supports 3-pin DC fan control you can plug the existing CG fans into your mobo. I don’t think the full build mobos support DC fan control (but could be wrong).
c) Unscrew and replace the 120mm CG fans with non-RGB PWM fans of your choosing from Noctua, Arctic or some other brand. In this case you will be undoing the cable management of the long fan tails that HL has done for the CG fans. Run your fan’s own tails under the backplane or off to the side from the front based on whatever your level of patience and OCD for cable management is. To connect 6 fans you will likely need some sort of hub, fan-out or daisy-chain cable like one of these;
https://forum.45homelab.com/t/i-recommend-this-100-fan-mod/958/25?u=digitalgarden
d) Unscrew and replace the 120mm CG fans with RGB fans. I can’t help with the details of this, but TechnoTim’s video might help. You will probably need to get some sort of RGB or ARGB controller depending on the RGB capabilities of your motherboard and what inputs you want controlling the RGB.
If you leave the fan connectors from the PDB disconnected (b, c, d), you should probably put some electrical tape around them to prevent a short, and tuck them away.
Put the mid chassis fan wall back in with its screws and the front faceplate back on with its screws. They are probably the same screw, but I don’t remember.