Go to your profile look to the top right and you will see avatar click on that.
Next the page comes up there you can change your avatar,Just go to gallary and pick the one thats best for you and click in the little o and then go to bottom and click save.
But before you pick one make sure to clear the one you have.So the one you pick can go there.
If you need more help with it go to the FAQ'S page.