ASUS began releasing Android 4.2 Jelly Bean update for Transformer Pad 300 in early March, and it is the first non-Nexus tablet to receive one. ASUS released a full 4.2.1 ROM for the TF300 available via OTA or direct download.
For users of the Transformer Par 300 who had unlocked the bootloaders of their device and missed the Over-the-Air update, they have the option to download the official ROM directly from ASUS' servers.
Be reminded that the upgrade is available for three distinct variants - Global, China, and P2P. Make sure that your handset matches the ROM variant before flashing it into your device. The Android 4.2.1 Jelly Bean update sizes up to 500MB, and Wi-Fi connection is preferred for stable connection and fast download.
The 4.2.1 ROM includes several enhancements, bug fixes, performance improvements, lock screen widgets, quick settings, multi-users and many more. Download the official ROM from ASUS Support.
In case you do not want ASUS version of the update, another alternative is available for owners who like to experiment and tinker. CyanogenMod developers have released their own Android 4.2.1 Jelly Bean update, and they are doing a good job, making sure that happiness comes with their custom ROM.
CM10.1 for ASUS TF300 is still a Nightly build and 162MB in size. You can it from their Web site, CyanogenMod Downloads.