Originally Posted by ivan_nik
Download link for update files for Apollo 3.2.5 did not work. I cann't download install.sh file. PM to me or post new link to it please.
This procedure is obsolete any any xx.3.x.x version of FireOS can be rooted with Towelroot. From there you can unlock the bootloader, install TWRP and flash any custom rom for this device including KitKat, Lollipop and Marshmallow variants. No need to rollback or mess with Safestrap.
The above statements may counter some older guidance you see in the forums. The 'discovery' that the bootloader can be unlocked on any 3rd gen device is fairly recent.
After rooting be sure to keep the device network isolated (WiFi off; Airplane/flight mode engaged) to prevent an unwanted OTA update from Amazon that may ruin your day.
See the following thread to unlock the bootloader. You'll benefit from some technical background as the guide is written for experienced users.