Here is the Android 4.2 Beta firmware for ClearTab SR-77. You can download the firmware here:
How to flash firmware?
1 Prepare an empty micro sd card; (just have no other content in the micro sd card)
2 Download the above firmware to your Computer ( NOT TO tablet!!)
3 Decompress the file you just downloaded in your PC, and you will get the following files:
Copy all of those file to the root directory of your micro sd card.
4 Power off your ClearTab SR-77 tablet pc. insert the micro sd card, press the VOL- and power button together till you see the upgrade mode start:
5 Wait the Upgrading to finish itself, then T7 will shutdown automatically.
Here is the screenshot after you made the upgrade: