The Forms toolbar contains the Combo box control.

Select the Combo Box, draw it on the worksheet so that it covers cell F1

Right click on the box, and select Format Control...  From the Control tab, specify the Input Range as XRange, and the Cell Link as $F$1 (remember, it is easier and less error prone to click the cell with the mouse rather than type in the cell address).

Click anywhere in the worksheet so that the drop-down box no longer has the handles (squares at the corners and the centers of the edges) around it.  Now it is ready for use.  Select any month as the starting month and watch the chart change.