sudo apt-get update not working

Hi, I want to run sudo apt-get update not working in my SSH, but it shows

Ign:1 http://mirrors.digitalocean.com/ubuntu disco InRelease
Ign:2 http://mirrors.digitalocean.com/ubuntu disco-updates InRelease
Ign:3 http://mirrors.digitalocean.com/ubuntu disco-backports InRelease
Err:4 http://mirrors.digitalocean.com/ubuntu disco Release
  404  Not Found [IP: 172.67.148.71 80]
Err:5 http://mirrors.digitalocean.com/ubuntu disco-updates Release
  404  Not Found [IP: 172.67.148.71 80]
Err:6 http://mirrors.digitalocean.com/ubuntu disco-backports Release
  404  Not Found [IP: 172.67.148.71 80]
Ign:7 http://security.ubuntu.com/ubuntu disco-security InRelease
Err:8 http://security.ubuntu.com/ubuntu disco-security Release
  404  Not Found [IP: 91.189.91.38 80]
Reading package lists... Done
E: The repository 'http://mirrors.digitalocean.com/ubuntu disco Release' no longer has 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.
E: The repository 'http://mirrors.digitalocean.com/ubuntu disco-updates Release' no longer has 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.
E: The repository 'http://mirrors.digitalocean.com/ubuntu disco-backports Release' no longer has 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.
E: The repository 'http://security.ubuntu.com/ubuntu disco-security Release' no longer has 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.

How do I solve this issue?

Ubuntu version -> 19.04

Thank you