[Q]Trying to flash to stock ROM to update bootloader

I want to update my cyanogenmod to lollipop, but the boot-loader isn't supported, so my understanding is that I need to flash to stock, get the ota to update the boot-loader, then I can flash to cyanogenmod. When I try to boot into the stock rom images I have downloaded, I get a red triangle. What am I doing wrong? I am s-on.