how to upgrade to latest version?

Manfred Hampl suggests this article as an answer to your question:
FAQ #3037: “no rolling release”.

"Ubuntu 18" does not exist. There are two different releases - Ubuntu 18.04 from April 2018 and Ubuntu 18.20 from October 2018. Ubuntu 18.10 is not supported anymore, so I assume that you are talking about Ubuntu 18.04 LTS (long time support)

thawab version 4 is not available for Ubuntu 18.04, but only for newer Ubuntu releases (e.g. 19.10).

Which is correct?

thawab

thwab

thawab is not in in Synaptic Package Manager, thwab 4.1.1 is.

Perhaps there is a PPA. Obviously this comes with the caveats of a PPA

From the control file for thawab 4.1-1:

Package: thawab
...
Homepage: http://thawab.ojuba.org
Description: Arabic/Islamic encyclopedia system
 Thwab is an Islamic literature library which features:
...

My assumption is that "Thwab" is a typo error.

When checking the prerequisites, it seems that all dependencies for thawab 4.1-1 (for disco) can be fulfilled on Ubuntu bionic.
I assume that the following will work on Ubuntu 18.04:
Download https://launchpad.net/ubuntu/+archive/primary/+files/thawab_4.1-1_all.deb
manually install it with "sudo dpkg -i thawab_4.1-1_all.deb"
install missing dependencies (if required) with "sudo apt install -f"

 4.1-1 on Ubuntu 18.04 installs and imports the .bok books into the program well.
But when i wanted to do a search, and tried, instead of typing directly in Arabic, to copy-paste the word I needed from my browser (with this useful tool called yamli) the program simply froze rather than pasting...
Then when trying a search, my first option gave 0 results (which I doubt is exact) and after that it was impossible to make a new search. I would type the new query and press return but nothing would happen.