Updating from such a repository can't be done securely, and is therefore disabled by default(For Kubuntu 19.04))
1520
How to install software or upgrade from an old unsupported release?
339
How to resolve "Failed to download repository information"?
24603
apt-get upgrade gives "403 forbidden" error
24165
Upgrade to version newer than repositories
144022
How to install a package from a local repository?
46424
Cannot update software center repositories
133
How can I fix a 404 Error when using a PPA or updating my package lists?
12054
How do I deal with "unauthenticated sources" errors in the Software Center?
113379
Updating the repository isn't working
117112
Why isn't Ubuntu moving its repositories to old-releases anymore?
124649
E: Repository 'http://security.ubuntu.com/ubuntu bionic-security InRelease' changed its 'Suite' value from 'bionic-updates' to 'bionic-security'
94171
Why Does Software Updater Say No updates Available, but apt-get upgrade Shows updates available?
56150
Can't upgrade/update ubuntu 18.04 as apt/dpkg error is showing up
743
How do I enable the "Universe" repository?
112879
W: The repository 'http://download.fpcomplete.com/ubuntu yakkety Release' is not signed
59768
How do I enable the "multiverse" repository?
13107
How to install updates via command line?