It has almost been two weeks since the release of the Apple TV 4.2 software update, and now based on iOS 4.3, Apple has just rolled out a minor Apple TV 4.2.1 update to fix screen flicker issues as well as problem arising when waking from sleep and the audio output.
The official changelog from Apple's support document:
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.
How To Update Apple TV To 4.2.1
To download Apple TV 4.2.1 software update, head over to Settings and then General on your Apple TV. Click on Update Software and then on Download and Install to update your Apple TV to the latest 4.2.1 software.
Alternatively, you can also manually download the software from the link given below and update your Apple TV to 4.2.1 using iTunes.
Download Apple TV 4.2.1