You will need to use the web app to do so. If you're in the mobile app, you can use More (the three dots) menu and then 'Web Version' to access it easily. There should be a link to your player's profile on the web dashboard which will have an "upload photo" button. If the upload button is not available, then your club has elected to limit permissions to change photos.
To update a player's recruiting photo, please refer to a separate topic.