I am running Lubuntu 18.04 on my computer. However when I go to “Software and Updates,” in the “other software” tab, it still lists “Ubuntu 17.10 Artful Aardvark (Source Code) Community-maintained” and “Ubuntu 17.10 Artful Aardvark (Source Code) Restricted software” and NOT anything for 18.04.
Assuming it’s preceeded by “cdrom,” then that’s probably because you installed from that initially. In general, “Other Software” should be otherwise blank unless you’ve got some special fancy non-normal and probably unsupported repositories.
It may not have “cdrom” in front of it, but I completely agree with @wxl’s suggestion.
My box is dual boot (Lubuntu 18.04 & hirsute). This hirsute (what will be 21.04 when released) system I’m using has
which are there because this system was installed in the artful cycle (it’s been upgraded every 6 months since then and is now hirsute)
You’ll note no check-boxes for any of them, I could remove the useless sources, but I like the reminder of when I installed this system (my bionic system (dual-boot) is much older)
Addendum: Sorry, yours will look different to this, as I’m using modern LXQt and not legacy LXDE, but I’d still expect something like this image.