You must have adb(android debugging bridge on your pc open command window in the adb folder( hold the shift key and right click on the adb folder for command prompt) connect your Android phone to your pc and type adb device(s) and enter key it will bring the device serial number eg 123456etc type adb shell and enter it will bring something like # then type cd /data/system/ press enter it will display sh/data/system/ then type rm password.key that all.