If you have a “mobi” format eBook file and want to convert it to “epub” format like me, you can use the software “Calibre” to do it, which is available on Linux/Mac/Windows.
For Ubuntu, run the following commands to install Calibre and convert the format.
sudo apt-get intall calibre
There are also online services which can do the same thing like here. You can use such services if you don’t want to install a software to do it.