Over time the monitor screen may build up a layer of dust, debris, even fingerprints and smudge marks from handling the monitor. Here's the instructions to clean the monitor screen:

First, turn off the monitor.

Second, use a screen cleaning brush to dust off the larger particles on the display area. This step is vital to avoid any scratches or smears as you continue the cleaning process.

After the dust and debris have been removed successfully, take a dry microfiber cloth or cleaning tissue to gently wipe the surface. Do not place too much force on the screen as you could risk damaging the liquid crystal display. 

For more noticeable smudges or oily residue, spray a small amount of screen cleaner onto the cloth or tissue and wipe the area. It is not recommended to use water for cleaning, as this may easily leave water spots on the screen. Use screen cleaner for electronics only; and check with the manufacturer for LCD monitor application.  

Isopropyl alcohol or household cleaning products are not recommended as they might damage the top coating and utterly ruin the display. Do not spray the monitor cleaner directly onto the monitor screen, as the cleaner may seep inside the monitor and cause damage.