After creating your avatar you can edit it if you want.
For editting your avatar you can follow these steps;
1- Open your Facebook application.
2- Click on the Settings, at the top right corner of the screen.
3- Click on “See More”
4- Click on Avatar.
5- On the right side of your screen click on Edit key.
6- Modify your Avatar.
7- Approve your modifications and follow these steps to finish your job and enjoy your new avatar on Facebook.