To your profile button Go . Then your avatar button. If you want to upload one you have saved on your computer click browse and it will take you to your computer and from there you can go find your pictures, wherever you keep them. Double click the picture and hit save. Like all things you may need to resize your pic if it's too large.
You can grab one from the net, just right click the image click properties , go to the Url and select all and copy. Then paste it in either of the next two boxes (sometimes if one won't take it the other will0.
Or you can select an Avatar from the gallery
button below the last two boxes and it will take you there and just click the avatar you want and hit save. I will demonstrate and give you one now. But you can easily change it by following the instructions I just gave you,