Configuring Proxy Settings Manually Ubuntu

Steps:

sudo gedit /etc/apt/apt.conf &

then inside apt.conf file write your proxy settings in the following format:
for http:
Acquire::http::Proxy "http://proxy_server_here:port_here";

for ftp: example
Acquire::ftp::Proxy "ftp://10.200.100.100:8080";

If you want to change this, just remove the lines we just wrote.

have fun