Like I said, for a Pocketbook you should just download the specific pocketbook release from github, then unpack it to the device storage.
If you want a simple script to install it from the device (connected to WIFI obvs and reboot after)....
Code:
#!/bin/sh
tmpDir=/var/tmp/koreader
rm -rf $tmpDir; mkdir $tmpDir; cd $tmpDir
curl -s https://api.github.com/repos/koreader/koreader/releases/latest \
| grep -wo "https.*pocketbook.*zip" | wget -qi -
unzip $tmpDir/$(ls) -d /mnt/ext1