How do I change my update server in Ubuntu 18.04?

The first thing I did after installing 16.04 LTS was to change my update server to Main Server. I can not find a similar option in Ubuntu 18.04.

How do I change it?

Open the terminal and type software-properties-gtk . From the opened window, open the drop down menu in front of Download from: and select main server

You can open the Software & Updates app in Ubuntu and change and change Download from to the Main Server as mentioned @Eskander:

If you are on KDE you don't need software-properties-gtk instead you can use Muon Package Manager which you can find from Application launcher.

Then just go to Muon Package Manager->settings->configure software sources and then select your desired server(In this case main server).