Today I tried to upgrade my Ubuntu version from 17.04 to 18.04 following this tutorial, but this didn't work:
$ sudo apt-get update
Ign.:1 http://pl.archive.ubuntu.com/ubuntu bonic InRelease
Ign.:2 http://pl.archive.ubuntu.com/ubuntu bonic-updates InRelease
Ign.:3 http://pl.archive.ubuntu.com/ubuntu bonic-backports InRelease
Ign.:4 http://pl.archive.ubuntu.com/ubuntu bonic Release
Ign.:5 http://security.ubuntu.com/ubuntu bonic-security InRelease
Ign.:6 http://pl.archive.ubuntu.com/ubuntu bonic-updates Release
...
I followed all steps, but sudo apt-get update
gives me these errors. How can I revert these changes to first update to Artful and then Bionic?