To change your flight there are some restrictions. Read more about them and follow the instructions at the link  http://www.tam.com.br/b2c/vgn/v/index.jsp?vgnextoid=ee7decacb8a8a210VgnVCM1000009508020aRCRD and change your flight date.
If you have checked in online, you will need to cancel the service to carry out the alteration procedure; click here and see how to cancel your check-in carried out over the internet and using smart phones.