In any case before installing newer firmware you should probably first install the newest version of hotfix / jailbreak bridge installer from the snapshots thread:
https://www.mobileread.com/forums/sh...d.php?t=225030
And do extra 1-2 reboots after installing that. Then install Amazon's update bin, and then again do ~2 extra reboots to have everything jailbreak bridge related restored.
IIRC Backdoor Lock and/or "folder tricks" only stop Kindle form downloading OTA bin and don't stop you from downloading newer firmware update bin file on you computer from Amazon's website, copying that to your Kindle and then using "Update your Kindle" option in menu to install the update.
Also you should know that after installing latest firmware those "folder tricks" will no longer stop future OTA updates. Those tricks no longer work since 5.12.x IIRC.
p.s. I'm curios, do you actually want to have that mess of new UI that comes with fw 5.14.2?