$ ll /var/lib/apt/lists/*ewton*
ls: cannot access '/var/lib/apt/lists/*ewton*': No such file or directory
$ sudo apt-get update
Hit:1 http://archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://security.ubuntu.com/ubuntu xenial-security InRelease
Hit:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:4 http://archive.ubuntu.com/ubuntu xenial-backports InRelease
Ign:5 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton InRelease
Get:6 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton Release [7.881 B]
Get:7 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton Release.gpg [543 B]
Get:8 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton/main amd64 Packages [119 kB]
Fetched 128 kB in 0s (235 kB/s)
Reading package lists... Done
$ ll /var/lib/apt/lists/*newton*
-rw-r--r-- 1 root root 550763 Jan 18 11:45 /var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_main_binary-amd64_Packages
-rw-r--r-- 1 root root 7881 Jan 18 11:45 /var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_Release
-rw-r--r-- 1 root root 543 Jan 18 11:45 /var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_Release.gpg
Try to force your case
$ sudo rm /var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_Release.gpg
$ sudo apt-get updateIgn:1 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton InRelease
Hit:2 http://archive.ubuntu.com/ubuntu xenial InRelease
Hit:3 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton Release
Hit:4 http://archive.ubuntu.com/ubuntu xenial-updates InRelease
Get:5 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton Release.gpg [543 B]
Hit:6 http://archive.ubuntu.com/ubuntu xenial-backports InRelease
Hit:7 http://security.ubuntu.com/ubuntu xenial-security InRelease
Fetched 543 B in 0s (1.308 B/s)
Reading package lists... Done
$ ll /var/lib/apt/lists/*newton*-rw-r--r-- 1 root root 550763 Jan 18 11:45 /var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_main_binary-amd64_Packages
-rw-r--r-- 1 root root 7881 Jan 18 11:45 /var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_Release