Whatsapp has started Payments feature in India based on UPI against apps like PayTM, Google Tez etc. It makes easier to transfer money through whatsapp. In order to use Whatsapp Payments you need the updated version whatsapp, so update your whatsapp to the latest version available. For now Payments need to be activated by other whatsapp user by initiating a payment request who has payments already enabled, So, it is kind of an invite system.
Once Payments is activated for you, you can go into settings to find a new option named payments.
You can now proceed to set up payments on your whatsapp to transfer or accept money on whatsapp. Make sure you're connected to internet and it will prompt you to select your bank form the list of banks.
After selecting the bank, you will need to verify the OTP sent to you, So, your whatsapp number also need to be linked to the bank account for UPI to work. After verification of OTP, the bank will be added and you can proceed to setup you UPI PIN of 6 digits.
Now that everything is set, you can open any conversation and open the paper clip option to see a new payments option and send and recieve money on whatsapp.
You can also send money to any bank account using UPI from payments page either by entering the UPI ID directly or scanning the QR code of the UPI ID of the reciever.