The Apple iMac is an all-in-one system whose display mounts on a pedestal. Out of the box, Apple's iMac systems offer little latitude for adjusting the height of the display. If you must lower the screen height of an iMac, it may be easier to adjust other aspects of the workspace instead. Apple also sells a mounting kit that can be deployed to connect an iMac to a third-party monitor stand.
Adjust the angle of the iMac display. All iMac computers tilt, and adjusting the angle will slightly lower the screen height.
Deploy a desk with multiple levels. You may need to adjust the height of the surface your iMac rests upon. Look for a desk with a central platform that can be lowered.
Add a keyboard tray below your computer desk. If you are contending with obstructions above the iMac, this may be an option. By clearing space, you may be able to place your iMac near the front of your desk, in front of the obstructions.
Purchase the Apple Video Electronics Standards Association (VESA) mount, which will allow you to install the iMac on any standard VESA display mount. The Apple VESA mount is available for 24- and 27-inch iMacs.