Im newbie to Joomla. How do I change the main menu like screen below to my customize menu?
Create your own custom menu from the backend. Login to your administrator panel.
Menus->Menu Manager->Add New Manager
Enter the menu details and you will be able to see it in the drop down. Select menu type items and assign articles, contact etc to your newly created menu.
Now under Extensions tab click on Module Manager . Now manage your custom menu to the position you want it to display. Like if you want to display your custom menu as main menu then go to main menu and assign your menu to it under basic options and the position you want it to display.
Following are the links where you can find more help-