Well, download last nightly from here:
http://build.koreader.rocks/download/nightly/ At the moment it is this:
http://build.koreader.rocks/download...2018-03-28.zip (download zip version).
Don't go to last stable.
After that, copy koreader directory from zip to .adds directory on your device. And, after that you'll have option to start koreader.
EDIT: Ken was a bit faster then I.