Not sure if it's been posted here yet but the mega link for the firmware in the instruction PDF for flashing from 6.1 -> 8.1 appears to be dead. Found this mirror link on gdrive that worked for me:
https://drive.google.com/uc?id=1BUDt...xport=download
On 8.1 now and it pretty quickly picked up on the OTA updates to get itself up to the current one right after. (Needed to do so in like 3 steps, I'm assuming those updates work incrementally or something.)