From your mobile phone:
To delete a profile picture:
go to "Profile" by clicking on the icon in the bottom right corner; click on the profile picture; select the picture you want to delete; click in the centre of the screen and then on the recycle bin icon in the bottom right corner.
click on the "Add photo" button to the left of the list of uploaded photos at the bottom of the screen;
select the photo you want to be your profile picture and click on the "Main photo" button in the bottom left corner.
select the new photo and click on the "Main photo" button in the bottom left corner.
From your computer:
Click on your profile picture in the top menu of the screen > select "My profile" > click on your photo on the left - a menu with your uploaded photos will appear > choose one of the actions under the photo: "Upload another photo", "Add more photos" and "Delete photo".
You can upload a photo from your computer, take a new one with your webcam, or choose it from a list of photos you have already uploaded.