Last night I attempted to upgrade 2.0.4.13 to 2.0.5.6 again. (If I count well this was about the 20th attempts.)
Seemingly the God do not want me this upgrade. I tried twice.
First, I accidentally cancelled the installation at around 35% (pressed CTRL+C in the wrong terminal window)
Second time my PC went blind at about 70%. I could see only a blank screen and an error message, and had no idea whether the install continued in the background or not. I waited half an hour trying to find workaround on my other phone but I finally had to reboot my PC.
After both of these attempts when I rebooted the phone, as @mautz suggested, I was able to connect to it from the PC with ssh.
However when I tried the 'version --dup' again, I got an error "Error message: Couldn't resolve host 'releases.jolla.com'"
I will try again. I think if my PC won't die in the middle of the process again, it will be success.
I have followed the instruction from here.