The mouse has a primary and secondary mouse button. You use the primary mouse button to select and click items, position the cursor in a document, and drag items. You use the secondary mouse button to display a menu of tasks or options that change depending on where you click. The primary mouse button is normally the left button on the mouse. For accessibility needs, or for leftâ€'handed mouse use, you can reverse the buttons and use the right mouse button as the primary button. This procedure tells you how to reverse the function of the right and left mouse buttons through Mouse Properties in Control Panel.

To reverse the function of the right and left mouse buttons:

1. On the Start menu select Control Panel

2.In Control Panel:

-Be sure you are in Classic View (all Control Panel icons are showing).
-If not, under Control Panel in the left pane, select Switch to Classic View.
-Select Mouse.

3.In the Mouse Properties dialog box, on the Buttons tab, under Button configuration:

-Select the Switch primary and secondary buttons check box.
-Select OK.

4.To close Control Panel select the Close button.

Source: microsoft.com