Theres a couple ways safest i've found is normal download and update. As far as ota if your unlocked no luck there but you can download from asus website place on root of sd or internal storage and wait for triangle to come up in notification back like as if ota. I would go through each update starting from where you left of before flashing cm especially going to 10.4.2.20 to 10.6.1.8 as it uses 10.4.2.20 as base for update from there. Remember once on jb bootloader your stuck there cant downgrade back to ics and theres some major changes to 10.6.1.8 and would have to reroot once get there but with the changes becarful as to what recoveries and roms u use once on 4.2.1 u can only use roms based on that and have to use the twrp 2.4.4.0 patched recovery but there are also alot of unknowns as to asus' new update so caution read and ask before flashing because many are bricking with no fix except nvflash right now. So also i would do and nvflash back up as if something should go wrong its your best and only way outnonce on 10.6.1.8 but back up before going through all this because as of now nvflash isnt available for jb 4.2.1. Goodluck and becarful.
Heresna good thread to read and ask questions on how to go about getting to 10.6.1.8 aka 4.2.1 most of all read and ask before getting into a pickle.
http://forum.xda-developers.com/showthread.php?t=2187982
I'll second this advice. I just upgraded my tablet from ICS yesterday, too, and this is exactly what I did. It worked great. I stopped at the last 4.1 update, though, because CM won't work on 4.2 yet.
And I second^2 the advice about setting up NVFlash before flashing the Jellybean update. I did that, too. Of course, I was doubly motivated by the fact that I had just recovered from a seven-month soft-brick (which was why I was still on ICS).
Anyway, good luck, and enjoy the Jellybean update.
Putting the ILLA in KILLA since 1985
