Unfortunately the option is only available on the desktop version of the site at the moment, but if the browser you're using has a "Desktop mode" option (most do), that should do the trick. Once you enable that, you can head to your profile and click the button below your profile picture.
If you tap the side bar on the left there should be a thing that says go to full website or something. tap it then tap the 3 lined bar on the top and tap profile. It should say change profile picture by your profile picture. Then change it and at the bottom press save