In order to change and edit your profile picture, simply visit the profile tab and click on the profile picture. This will open a screen where you can add a picture and crop it so it fits the way you like it.
You can decide to take a new picture for yourself or add an existing picture from your media library.
Another way to change your profile picture is to click on the settings wheel on top of the profile tab. This will bring you to your settings where you can edit your picture and add more information to your settings as well as setting up your preferences.