I only have the HT100 so I can't comment on the other amps, but I believe it's the same across the board.
So, start by taking the screws of the back and bottom. Note that there's 6 on teh bottom unlike other amps that only have 4. Yet another example of Blackstars "belt and braces" approach.
Anyway, once you're done, lift out the chassis by reaching in and holding the 2 transformers on either side. Lay it down BUT DO NOT
lean it on the valves. I prop it on a bench edge so the tubes are hanging free. Up to you how you do it - but watch the valves.
Then you need to ensure there's a resistive load on the amp. In other words, plug in your speaker cab.
The way Blackstar have designed the amp means that you have to have a cable plugged in to bias/operate the amp. I use a spare George L patch plug. Nice and neat and doesn't leave a cable on teh floor.