What is the easiest way to update/upgrade an offline Ubuntu machine with the help of a Windows machine that has internet access?
It provides a graphical interface for gathering updates, packages, and dependencies for offline computers. It distinguishes the "suites" (backport, multimedia, volatile, stable, testing), and allow you to mix the suites.
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There are some complications getting apt-offline to work properly on Lubuntu and all other methods seem to be too old and deprecated: Apt Off - last update 2006 Apt Medium - last update 2007 Keryx - last update 2011, looks like this project was discontinued I hope I will be able to somehow update the offline machine, finally.
While I was still not able to update/upgrade the offline system due to authentication errors, I accepted an answer, because it's taking too long.