How do I replace old avatar with new one?

I've searched for instructions on this, but when I upload my new avatar I see no way to delete the old one or replace it with the new one. There's a slot for picture and a slot for image. When I click the Select button it doesn't let me select one of them, it just wants me to upload another one.

Hover over your current avatar at the top right hand side of messages and notifications -> Settings ->Scroll down to Avatar and backgrounds and make your changes there.