Just click on 'User CP' near the top of the screen (can only be done when accessing the site from the web not the app). A side-panel will appear that enables you to change any of your personal details - including your avatar.
Go into photos on FB and upload the photo from either your camera roll or you can take a photo and upload it to FB. Once the photo is uploaded press on it and then press the tab top right and choose Make Profile Picture.