I just went through this recently, so hopefully this will help. Follow the steps I’ve outlined in this post.
It’s annoying somewhat because you have to go through the install/setup procedure twice - first when you boot from the new SD image (download here), and yet another time when it’s been installed on the eMMC.
Note: I don’t know if my system is just buggy or something, but it did have issues booting up correctly on Step 5, after I’ve installed on the eMMC. I found that if I shut down completely (instead of reboot) and turn on the machine, I had better luck getting it to load from the eMMC that first time. YMMV.
Note 2: FWIW, I also experienced the black screen on boot w/LED light lit issue after a recent Testing Update. The reason I had to do a re-install was because I could not figure out a way to recover from that. If you find a root cause, I’d be curious to know what it was as well.