1- Open WhatsApp and hit the menu button (three vertical dots on the top-right) >> Settings >> Chat Settings >> Backup conversations
2- This file will be stored as "msgstore.db.crypt7" in your phone's
/WhatsApp/Databases folder. WhatsApp recommends that your rename this file to "msgstore.db.crypt7.current", without the quotes, to make it easy to find when you want to restore the backup.
3- To restore conversations from a backup, uninstall WhatsApp and find the correct backup file from the WhatsApp folder. Slightly older backups are named "msgstore-YYYY-MM-DD.1.db.crypt7". In order to restore any of these, rename the file to "msgstore.db.crypt7".
4- Now reinstall WhatsApp. Once you verify your phone number, WhatsApp will show a prompt saying that it has found backed up messages. Tap Restore, choose the correct backup file and wait for the conversations to appear in the app.