To change the main photo on your profile, go to the "My profile" section by clicking your screen name in the top right-hand corner of any website page. Then scroll down to the bottom of the page, where you will find you photogallery section.  
To set profile or cover photo from existing photos
  • Move mouse over any photos in your photo gallery, and you will see icon/links for setting your photo as profile, as a cover photo
  • After clicking on according action you will be able crop,zoom or rotate your photo according to requirments for profile or cover photo.   
To upload new photo as a profile or cover photo 
  • Add new your photo(s) by clicking to the Computer of Facebook icons.
  • Select a photos from your computer, or your Facebook account you want to upload and have as a profile/cover photo
  • Wait for the photo to be uploaded,
  • Move mouse over photo you just uploaded and you will see icon/links for setting your photo as profile, as a cover photo
  • After clicking on according action you will be able crop,zoom or rotate your photo according to requirments for profile or cover photo.