If you’re trying to update Luma3DS to anything newer than 7.0.5 it won’t work. A9LH should be updated to B9S (boot9strap) for the update system to work again.
If you’re on a verison of Luma before version 7.0.0 you need to manually update to 7.0.5. Then, follow the official guide given at https://3ds.guide/updating-to-boot9strap. The guide looks long, but you will be going through it really quick. It’s all just setup for your SD card to get started and the guide is trying to be idiot proof.
They are asking that no one make a video guide or if there are some out there, do not follow them as there is a risk of bricking your console.