My iMac, has a dead pixels. What should I do ?

Take it back to the store where you bought it - if you bought it online contact AppleCare. How Apple deals with dead pixels depends on how many and where they are. If you bought direct from Apple, online or in an Apple store you may return the computer within 14 days for a full refund.

Have a look at > Is this a dead pixel?: Apple Support Communities

and > About LCD display pixel anomalies for Apple products released in 2010 and later