When you noticed your monitor is looking yellowish and its screen to be difficult to read then you can understand about inner side of your monitor as dirty as outside.
Never place your monitor on the ground, always use computer table or shelf for this purpose.
First unplugs your monitor power from the electrical outlet.
Never open the cover of the monitor. First use the compressed air and start clean from back to front of your monitor case.
Use lightly dampened cloth or spray any cleaning fluids on the monitor body and use paper towel to dry it. Do not spray any fluids near the ventilation hole on the monitor. Use any glass cleaner to clean the monitor screen with soft cotton cloth.