Simple, just move the mouse to the edge of the taskbar, you should see it change to a double arrow, click and drag down until it's one line. It may be necessary to click on an empty part of the taskbar and select the "Lock Taskbar" option.
My taskbar has become so wide and loaded with icons that it interferes with screen content How can I reset it to one line?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic