The first thing you'll need to to is open the update manager. These instructions to help you do this in Unity.
Go to the Unity Launcher, Select the Circle of Friends Icon, type in Update-Manager, either arrow down
(and then click enter) to it using the keyboard, or select it with your mouse or trackpad.
In the lower left hand corner you should see a "Settings" button. Select that.
The Update Manager Software Sources tabbed window should open. Click on the Ubuntu Software Tab.
Just more than halfway down you'll see "Download from:" with a setting. In my example it shows the URL: http://mirror.cc.columbita.edu/pub/linux/ubuntu'archive
To the right of the url, you'll notice a small down symbol as this is a drop down box. Expand that drop down and select "Other".
A "Choose a Download Server" window will appear. On the righthand side of the window, you'll see a "Select Best Server" button. Click on that.
Your system will begin a test to see what are the best mirrors available for you based on your network and location.
Once this is narrowed down and the mirror test selects a server, You will need to select "Choose Server" to set it.
You will get prompted for your account password to 'authorize' the setting of the newer mirror to your apt sources. This will update your apt sources to use the mirror that you just now chose.
Click Close, then you will see the following appear:
Click "Check" so that you update manager can use the new mirror and update the software that is available to you, such as both updates, and new applications that are now available by using this mirror.
It's important to note that whatever you set is now the default. If you happen to visit another part of the country it would be good to run this in order to set a better preference for the new location your at.