Ubuntu sudo apt get update 404 Not Found problem

I am using ubuntu 20.04.3 LTS on my system .
I’ve an error for 2 days. I couldnt update-upgrade or package install on my system I get below errors:

Hit:1 http://packages.microsoft.com/repos/code stable InRelease                                                                   
Hit:2 http://archive.canonical.com/ubuntu focal InRelease                                                                         
Hit:3 http://ppa.launchpad.net/deadsnakes/ppa/ubuntu focal InRelease                                                              
Hit:4 http://dl.google.com/linux/chrome/deb stable InRelease                                                                      
Ign:5 http://old-releases.ubuntu.com/ubuntu focal InRelease                                                                       
Ign:6 http://old-releases.ubuntu.com/ubuntu focal-updates InRelease                                                               
Hit:7 http://ppa.launchpad.net/kicad/kicad-5.1-releases/ubuntu focal InRelease                                                    
Hit:8 https://deb.opera.com/opera-stable stable InRelease                                                                         
Hit:9 https://packages.microsoft.com/repos/vscode stable InRelease                                                                
Ign:10 http://old-releases.ubuntu.com/ubuntu focal-backports InRelease                                                            
Ign:11 http://ppa.launchpad.net/team-gcc-arm-embedded/ppa/ubuntu focal InRelease                                           
Ign:12 http://old-releases.ubuntu.com/ubuntu focal-security InRelease                                                      
Err:14 http://old-releases.ubuntu.com/ubuntu focal Release                                                                        
  404  Not Found [IP: 91.189.88.247 80]
Err:15 http://ppa.launchpad.net/team-gcc-arm-embedded/ppa/ubuntu focal Release                                                    
  404  Not Found [IP: 91.189.95.85 80]
Err:16 http://old-releases.ubuntu.com/ubuntu focal-updates Release                                                           
  404  Not Found [IP: 91.189.88.247 80]
Hit:17 https://linux-clients.seafile.com/seafile-deb/focal stable InRelease                
Err:18 http://old-releases.ubuntu.com/ubuntu focal-backports Release 
  404  Not Found [IP: 91.189.88.247 80]
Err:19 http://old-releases.ubuntu.com/ubuntu focal-security Release  
  404  Not Found [IP: 91.189.88.247 80]
Ign:13 https://launchpad.net/~terry.guo/+archive/gcc-arm-embedded focal InRelease
Err:20 https://launchpad.net/~terry.guo/+archive/gcc-arm-embedded focal Release
  404  Not Found [IP: 91.189.89.222 443]
Reading package lists... Done

How can I solve this ?

>Solution :

Most of the repositories and PPAs in your sources.list are no longer available and are throwing errors. I’d recommend restoring the default repositories.

1. First, restore the default focal repositories using these commands:

   mkdir ~/solution
   cd ~/solution/
   wget https://gist.githubusercontent.com/ishad0w/788555191c7037e249a439542c53e170/raw/3822ba49241e6fd851ca1c1cbcc4d7e87382f484/sources.list
   sudo rm -r /etc/apt/sources.list
   sudo cp -r ~/solution/sources.list /etc/apt/sources.list
   

2. Remove all the PPAs in your system:

   sudo mv /etc/apt/sources.list.d/* ~/solution
   

3. Update the repositories:

   sudo apt update
   

Now there should be no errors.
Good Luck!