If you have bought a new phone and want to install WhatsApp, go to play store and install the WhatsApp application.
Enter your phone number. Whatsapp will do a verification. Follow these steps, so WhatsApp would verify you.
After verification, it will ask you to restore WhatsApp chat back or skip
Click on the “Restore” option and wait for some moments
Soon, WhatsApp will retrieve your old data.
This is the easiest way to do a WhatsApp backup.
Also learn “How to Reset Mobile?”