Lumia Software Recovery Tool
Ah :/ I had 8.1 on my PC already, so didn't have to download anything, and it's already reverted back now. Annoying that it seems you need to be online just for the program to let you install it when you already have it, but never mind, no big deal.
Hope the servers speed up soon.
Don't know if it'll be the same for you, but on my PC it's C:\ProgramData\Microsoft\Packages\Products, then \RM-1045 as it's for the 930. ProgramData is normally a hidden folder so you'll need to make sure they're displayed. Hope that helps!