The frequency bar position is adjusted by left click and drag of the up/down button.
The picture also shows the effect of zooming in, which is achieved by right click and drag anywhere on the bar except the up/down button. A left click and drag on the bar slides the display left or right across the entire range of the receiver (0 to 55 MHz). Dragging beyond the window edge continues to slide the display at a speed proportional to the distance away from the edge.