1. Goto Terminal and type sudo nautilus.
2. A folder window will appear, from there go to filesystem->etc->apt->apt.conf
3. Type the following
http://<proxy address> <username>:<password>@<proxy address>:<port number>
e.g. http://172.18.0.10 admin:firstname.lastname@example.org:3122
If you have any queries, do post below as comments….