Before starting make sure your new phone number can receive SMS and has an active data connection. Also make sure your old phone number is verified in WhatsApp.
1.Insert the new SIM card with the new number into your device.
2. Open WhatsApp.
3. Go to WhatsApp> Settings > Account > Change number.
4. Enter your old phone numberin the 1st box and your new phone number in the 2nd box and Click on done.
5. You will now be asked to verify your number, wherein you will have to enter the verification code sent to you on your new phone number, once done you are good to go.
Also, the group chat participants would be notified of your number change on the group but your contacts will not be individually informed.