Setting up apt-get to use a http-proxy

This is a temporary method that you can manually use each time you want to use apt-get through a http-proxy. This method is useful if you only want to temporarily use a http-proxy. 

Enter this line in the terminal prior to using apt-get (substitute your details for yourproxyaddress and proxyport).

export http_proxy=http://yourproxyaddress:proxyport

Done!!! 

Option 2:

Type in Terminal

gedit /etc/apt/apt.conf

(root authentication is required)

Now type this two lines in the text file:

Acquire::http::Proxy "http://username:password@proxyaddress:port";
Acquire::ftp::Proxy "ftp://username:password@proxyproxyaddress:port";

Save and Done!

Try this in terminal now, 

sudo apt-get update

You will see your ubuntu getting updated!