OK. First Of All Open Terminal (CTRL + ALT + T) and do the following command :
You will get a Window Like Below :
Now, What you have to do is go to a Free Proxy Site like free-proxy-list.net and get a Proxy IP and Port. You can choose the Proxy based on countries or anything else.
Click On Manual proxy configuration‘s radio button and choose Use the same proxy for all protocols. Then type in the Proxy IP address and port. Example :
Then Click on Close button. You will be asked for using the proxy completely on your system which means that the applications you install via syanptic or software-center use the Proxy mentioned. If you just want to trick a site, then ignore the dialog box. Here is an example of my IP details when my Proxy was changed :
If you have any doubts/suggestions/problems/feedback say it out in the comments, I will help you.