I don't know why Line 6 has to make this process so hard, their article about this is almost 2 years out of date and references software that, afaik, doesn't exist anymore. So I had the same issue where updating via iOS over bluetooth inexplicably failed, leaving my brand new Amplifi 75 in "recovery mode" (lights flashing). Trying to set my amp into "update mode" showed all lights lit solid (not all lights out as this article says). IGNORE THAT ARTICLE!!! Here is what you do: Go to http://line6.com/software Download the latest update of both the Line 6 Driver2 and the Line 6 Updater.
Install both. If you install the Updater without the Driver, the Updater will not see your device to update and it's totally not obvious what's wrong. Once you have the driver, it works peachy. In my case, I did not have to put the amp into update mode, it worked fine from the recovery mode it was starting up in. These instructions should work until Line 6 renames their utilities again and/or revamps the procedure. I don't know why Line 6 can't do things like make their software intuitive or keep their Knowledge Base up-to-date but from a long time user, they've been this way for years. I work IT doing some pretty complex stuff and even I'm mystified by their software design choices half the time. It's all over the place.
