Err:7 http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu artful Release
  404  Not Found
Hit:8 http://in.archive.ubuntu.com/ubuntu artful-backports InRelease
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu artful Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.

dat-asset-156@dat-asset-156-pc:~$ sudo apt-get update
Get:1 http://security.ubuntu.com/ubuntu artful-security InRelease [78.6 kB]
Ign:2 http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu artful InRelease
Hit:3 http://in.archive.ubuntu.com/ubuntu artful InRelease
Hit:4 https://repo.skype.com/deb stable InRelease
Hit:5 http://ppa.launchpad.net/webupd8team/sublime-text-3/ubuntu artful InRelease
Get:6 http://in.archive.ubuntu.com/ubuntu artful-updates InRelease [78.6 kB]
Err:7 http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu artful Release
  404  Not Found
Hit:8 http://in.archive.ubuntu.com/ubuntu artful-backports InRelease
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/n-muench/programs-ppa/ubuntu artful Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.