Solution:

1) Double-click on the desired graph object. (A striped border with handles appears around the chart.)
NOTE: If the chart is on a separate chart sheet in Excel, click the tab for the chart sheet and double-click on the chart. (A striped border with handles appears around the chart.)
2) Select the desired arrow to change. (Handles appear on the arrow.)
3) Select the 'Format' menu and select 'Selected Object...'. (The Format Object dialog box appears.)
4) Click the 'Patterns' tab.
5) Do one of the following:

a) To use the automatic line features, select the 'Automatic' radio button in the 'Line' group.
NOTE: 'None' from the 'Line' group is unavailable to arrow formatting.
b) To use custom features:

1] Select the 'Custom' radio button in the 'Line' group.
2] Select the desired arrow style from the 'Style' drop-down list box in the 'Line' group.
Line Style
3] Select the desired arrow color from the 'Color' drop-down list box in the 'Line' group.
Line Color
4] Select the desired arrow width from the 'Weight' drop-down list box in the 'Line' group.
Line Weight

6) Select the desired arrowhead style from the 'Style' drop-down list box in the 'Arrowhead' group.
Arrowhead Style
7) Select the desired arrowhead width from the 'Width' drop-down list box in the 'Arrowhead' group.
Arrowhead Width
8) Select the desired arrowhead length from the 'Length' drop-down list box in the 'Arrowhead' group.
Arrowhead Length
9) Click the Properties' tab.
10) Select the desired radio button from the 'Object Positioning' group.
11) Click 'OK'. (The arrow within the chart appears with the changes.)