In order to change the mirror we're using for updates, the procedure is to open the Software Centre, and go to Edit->Software Sources. There I can see the option "download from" and when I click it, the list of countries and servers appeared. I try to change it, but the problem is "choose server" button is disabled and can't be used. So, the only option for me was to click on "cancel" button or to close the window which means I gave up. What is the problem here, why I can't change the source for update and how to do that?
After you get to the country you want, you need to click on the little arrow to the left of the country name. That will open up the list of servers available int that country.
Here is a screenshot of the United States list opened, note the arrow on the left is pointed down as the list has been opened.
All you would need to do is to click on one of the listed servers and then box will be enabled and you can choose the server.
The Software Sources has a built in feature that will test all the servers and select the best one for your location. That will allow you to use the best server for you without having to test everyone manually. If you look at the upper right hand corner of the screenshot, you will see the button Select Best Server. Just click that and the test will start and find you the best server. This can take a few minutes so let it finish.
After either method of changing your server, you will need to update your package list with the info on the new server. To do that just open a terminal and run:
sudo apt-get update