Use LG's service tool. It will unlike OTA not brick yer phone, download the latest official firmware, for me it was 20h a couple of weeks ago, replace the recovery and un-root, keeping apps and settings.
The reason was to update baseband firmware. Then I needed ADB for root & recovery, and couldn't get it to work under Win 8.1 x64. It instantly worked on my netbook with Win7 x64.