You have last stable release, pretty old version.
Latest version you can found here:
http://build.koreader.rocks/download/nightly/
Then you choose latest nightly, at the moment it is this zip version for kobo:
http://build.koreader.rocks/download...3-g773e24f.zip
Also, you can found "released" releases here:
https://github.com/koreader/koreader/releases
When you download it, you install it by copying koreader directory on your device, but I guess you know that.
And OTA doesn't work for your version probably because OTA channels are changed some time ago. Or at least I think so, I don't do OTA. But, with latest version you should have working OTA also.