ubuntu apt update failed to fetch

When I do command sudo apt update, always get belowing errors:

Err:1 http://archive.ubuntu.com/ubuntu bionic InRelease
  Could not connect to archive.ubuntu.com:80 (91.189.91.23). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.149). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.152). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.161). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.162). - connect (111: Connection refused)
Err:2 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
  Unable to connect to archive.ubuntu.com:http:
Err:3 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
  Unable to connect to archive.ubuntu.com:http:
Err:4 http://security.ubuntu.com/ubuntu bionic-security InRelease
  Could not connect to security.ubuntu.com:80 (91.189.88.162). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.91.23). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.91.26). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.88.149). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.88.152). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.88.161). - connect (111: Connection refused)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease  Could not connect to archive.ubuntu.com:80 (91.189.91.23). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.149). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.152). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.161). - connect (111: Connection refused) Could not connect to archive.ubuntu.com:80 (91.189.88.162). - connect (111: Connection refused)
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  Unable to connect to archive.ubuntu.com:http:
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease  Unable to connect to archive.ubuntu.com:http:
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease  Could not connect to security.ubuntu.com:80 (91.189.88.162). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.91.23). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.91.26). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.88.149). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.88.152). - connect (111: Connection refused) Could not connect to security.ubuntu.com:80 (91.189.88.161). - connect (111: Connection refused)
W: Some index files failed to download. They have been ignored, or old ones used instead.

and I have done my proxy setting in file .bash_profile like:

function setproxy
{
      export {http,ftp}_proxy=xx.yy.com:80  
}

function unsetproxy
{
    unset {http,ftp}_proxy
}

setproxy

and check it with command env | grep -i proxy:

kay@cn:~$ env | grep -i proxy
http_proxy=xx.yy.com:80
ftp_proxy=xx.yy.com:80

At last, I found what I should do is to create an apt proxy conf file.

Apt loads all configuration files under /etc/apt/apt.conf.d. We can create a configuration specifically for our proxy there, keeping it separate from all other configurations.

Create a new configuration file named proxy.conf.
sudo touch /etc/apt/apt.conf.d/proxy.conf
Open the proxy.conf file in a text editor.
sudo vi /etc/apt/apt.conf.d/proxy.conf
Add the following line to set your HTTP proxy.
Acquire::http::Proxy "http://user:password@proxy.server:port/";
Add the following line to set your HTTPS proxy.
Acquire::https::Proxy "http://user:password@proxy.server:port/";
Save your changes and exit the text editor.

Your proxy settings will be applied the next time your run Apt.