1. Copy your backup to
a) internal storage/whatsapp/Databases (By default)
or
b) SDcard/whatsapp/Databases (Depending on the phones installation folder)
then Install Whatsapp from playstore
During the installation process, WhatsApp will automatically find your backup and ask you if you would like to restore it. Once restored, your old chats will reappear, on your new phone.
try it .. hope it will fix ....