What you just have to do is :
First of all ,Go to your Sd card then ,
SD Card–>Whatsapp–> Databases and look for two files named  msgstore-yyyy-mm-dd.1.db.crypt5 and msgtore.db.crypt.This file creates a record of all the dates on which you’ve sent or recieved messages in the last seven days , while second file contains all the message respective to that date .To restore your Whatsapp messages, rename msgtore.db.crypt toOLDmsgtore.db.crypt or something to identify the old backup easily . Select the database you wish to restore from the files left . After selecting rename the file to msgstore.db.cryptNow uninstall the whatsapp and reinstall it . It will find the database and ask if you want to restore the database , Click on Yes . The chat file you saved with the namemsgtore.db.crypt is restored. Restoring an database automatically clears your current chat history and there’s no way to merge your old database and your current chat history.