1. Go to System/Administration/Software Sources. You will be prompted with admin password to login.
2. In Software Sources window, make sure to activate source code option. Then change the server to the nearest one to your location.
In my case, I choose http://ubuntu.indika.net.id. Click "Choose Server" to go through.
3. Now, it's time to add another local repository mirror. Prepare the mirror list, and add it by highlighting each line, right click it, and choose "copy" option.
4. Go to Third Party Software Tab. Click "Add" button.
Then paste the line that has been copied before. do it exactly the same for another line.
After all mirror addresses are completely added, click the "Close" button.
You will see a window afterward, click "Reload" button
Soon you will see the mirror scanning in progress. You can see the download speed reaching approximately 90 Kbps because of the local mirror location factor. When it's finished, it mean that you have successfully adding local mirror to repository.
The second part of this how-to is a how-to get an update of an ubuntu application from local mirror repository.