ROOT LG L90

• Format sdcard using into ext2.

  mkfs.ext2 /dev/sdX1

• Download update.zip
  Move it to your ext2 sdcard
  
  
• Install Android SDK or
  
  • for Debian Wheezy:
    Add to /etc/apt/sources.list

    deb http://ftp.debian.org/debian wheezy-backports main 

    Update & install android-tools-adb

    sudo apt-get update && sudo apt-get install android-tools-adb

• Prepare your phone.
  Turn on USB debugging:
  Settings –> General –> Developer options –> USB debugging.
  
  If you don’t see it:
  Settings –> General –> About phone –> Common –> Software info
  – click 5-10 times on Build number.
• Connect phone & run Terminal:

  adb devices

  adb shell

  reboot recovery

• Disconnect phone, plug in sdcard
• In recovery choose "apply update from external sdcard" – update.zip
  Choose "reboot system now"
  
  The phone will reboot.

FROM
Profit!