The resolution of a Mac OS X Lion system can be changed using the option available as part of System Preferences. Click the Apple icon and select System Preferences from the menu list
In the System Preferences window, click the Displays icon under the Hardware section.
This would display the following Color window with option for changing the Resolutions. Now you can select your preferred Resolution from the list box.
If your nature of work requires frequent changes to the system resolution then it would be convenient to have the display option as part of menu bar. You can show the display option in the menu bar by marking the check box with label as Show displays in menu bar.