The 27-inch iMac comes with a high-resolution LED backlit widescreen display with a maximum resolution of 2560 by 1440 pixels. You can change the screen resolution of your iMac computer using the native option available on the Mac operating system. Higher screen resolution provides more room for opening multiple applications.
1). Click on the "Apple" menu in the top left corner of the screen, and then click "System Preferences."
2). Click "Displays" under the "Hardware" category.
3). Click the "Display" tab on the top. All of the resolutions supported by iMac are listed under "Resolutions." Select the resolution you want by clicking on it.