When users register to Whatsapp for the first time, they get a 1-year free trial period. After that, users have to pay $0.99 (or 0.89€ if you pay in Euros) to renew your license for one more year.
Follow the below steps to extend your WhatsApp. Expiry date.
Goto: SETTINGS=> ACCOUNT=> CHANGE NUMBER.
change your current number to any valid number that you have other than the number in which the account has been registered with.
Use the code recieved to verify.
And then change the number again to your previous number by entering the code you received from whatsapp.
you have another free year