To edit your profile picture, click on it. In the menu that opens, you may now have various options: You can upload a (new) picture, and change the shape and frame of your avatar, if you have already unlocked these options.
Profile pictures must comply with our Terms and Conditions, which will be displayed once again before you upload a file.
After uploading a new profile picture, you will have to wait 48 hours before you can upload another picture.
Further info about this can be found here.