MY LUMIA 920 WAS UPGRADED TO LUMIA BLACK.AFTER FEW MONTHS MY SCREEN FREEEZED AND AFTER FEW DAYS APPS ARE NOT RESPONDING.SELLER REPAIRED IT & HE TOLD ME NOT TO UPDATE PHONE.WHY THEY TOLD IT? I UPDATED PHONE AGAIN AND IT HAPPENED AGAIN.WHAT IS THE REASON?how can i solve it?