You can choose to do this process entirely on your device or with the help of a computer.
First, grab Amazon's latest update file from http://www.amazon.com/gp/help/custom...deId=201016350 (You can do this on your Kindle or your computer) then rename it from WHATEVER-THE-UPDATE-NAME-IS.bin to WHATEVER-THE-UPDATE-NAME-IS.bin.zip
Next, Backup all important data. What YOU classify important data as is all up to you. This could be apps (Free apps are located in "/data/app", while premium and paid apps are in "/data/app-asec". Please note that if you copy a premium app from "/data/app-asec", you must also copy it's corresponding folder in "/data/data".), family pictures from a recent vacation (Most likely located in "/sdcard/Pictures" somewhere.), or just some hard-to-find files that work on all newer android devices. You can choose to zip all the files up and put them on Dropbox, or use your computer to "adb pull" them. If you have any questions about backing things up, leave a comment down below.
Third step: If you downloaded Amazon's update file to your computer, you can transfer it over now or wait a step or two.
Now you need to reboot into TWRP recovery. Once you get there (Shouldn't take too long now, thanks to Hashcode! ), if you downloaded the file on your computer, you can wipe your internal storage (/sdcard), system, data, dalvik cache, and cache now. If not, wipe all above except for your internal storage (/sdcard) and flash the zip file you downloaded earlier then afterwards, wipe your Internal Storage (/sdcard). To those who just wiped everything and have the file downloaded on your computer, got to the "Advanced" menu in TWRP and hit "ADB Sideload". You will need to have the proper drivers if using Windows. (Windows always has drivers issues with Android. :/)
If you need help with anything in this thread, please leave a comment down below.
Now that all that is done, reboot your tablet and you should now be 100% stock with NO ROOT, TWRP REMOVED, ALL DATA REMOVED, ALL IMPORTANT FILES BACKED UP, and YOUR KINDLE FIRE HD 7" IS READY TO SELL.
Sent from my Amazon Kindle Fire HD using xda app-developers app