Apple TV 4.2.1 (Based On iOS 4.3) Update Released To Fix Screen Flickering Problem

Apple has just pushed a minor update for the second-generation Apple TV owners to fix a number of issues, including screen flickering problem which was reported by some Apple TV owners right after iOS 4.3 was released.

Full official change log is as follows:

TV compatibility
Addresses issues that may cause the screen to flicker or display incorrect color on some older TVs.
Wake from sleep
Addresses an issue where Apple TV may not wake up from sleep.
Audio
Addresses an issue where audio may not be heard on some TV models after switching from another input.
Stability and performance
Apple TV Software Update 4.2.1 includes stability and performance fixes for Apple TV.

To update your Apple TV, from Settings –> General , hit “Update Software”, followed by “Download and Install” to get to the latest build.
Optionally, you can also download the firmware file from the link given below and update Apple TV using iTunes.
Download Apple TV 4.2.1 (8F202)