In order to unlock the device, you will need to restore it to its factory condition. Firstly, you need to turn it off and to press and hold the Volume Up + Power buttons. When the Nokia logo appears on the screen, release the Power button and wait the for the Reset menu to appear. After that, using the volume keys select Wipe data/ Factory reset, press Power to confirm and restart the device after the process is complete. If you have issues, you can use Google search engine to find some video tutorials.