ubuntu 16.04.1 fix error for sudo apt-get update

Problem:
$ sudo apt-get update
..........
..........
Err:13 http://extras.ubuntu.com/ubuntu xenial/main Sources                                                        
  404  Not Found [IP: 91.189.92.152 80]
Ign:17 http://extras.ubuntu.com/ubuntu xenial/main amd64 Packages                                                 
Ign:20 http://extras.ubuntu.com/ubuntu xenial/main i386 Packages                                                  
Ign:22 http://extras.ubuntu.com/ubuntu xenial/main all Packages                                                   
Ign:23 http://extras.ubuntu.com/ubuntu xenial/main Translation-en_US                                              
Ign:24 http://extras.ubuntu.com/ubuntu xenial/main Translation-en                                                 
Ign:25 http://extras.ubuntu.com/ubuntu xenial/main amd64 DEP-11 Metadata                                          
Ign:26 http://extras.ubuntu.com/ubuntu xenial/main DEP-11 64x64 Icons                                             
Fetched 204 kB in 12s (16.5 kB/s)                                                                                 
Reading package lists... Done
W: The repository 'http://extras.ubuntu.com/ubuntu xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/xenial/main/source/Sources  404  Not Found [IP: 91.189.92.152 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.


Solutions 
1) Comment line inside /etc/apt/sources.list where http://extras.ubuntu.com/ubuntu
These packages are not actual ubuntu source so we can comment it out.

2) replace 
deb http://extras.ubuntu.com/ubuntu xenial main
deb-src http://extras.ubuntu.com/ubuntu xenial main

With
deb http://extras.ubuntu.com/ubuntu trusty main
deb-src http://extras.ubuntu.com/ubuntu trusty main

Since xenial package does not exist check here: http://extras.ubuntu.com/ubuntu/dists/
http://extras.ubuntu.com/ubuntu/dists/xenial/main/source/Sources

E: Problem executing scripts APT::Update::Post-Invoke-Success 'if /usr/bin/test -w /var/lib/command-not-found/ -a -e /usr/lib/cnf-update-db; then /usr/lib/cnf-update-db > /dev/null; fi'
E: Sub-process returned an error code
how can i fix it ??