Sign in to your Google+ Account and open your profile.
Click on Edit profile option.
Click on “change photo” just below your current display picture.
Select a photo from Your albums, From your phone, Photos of you, or from Profile photos. Or simply Upload a new photo.
Crop your picture, or resize it by clicking and dragging the corners. Click Edit photo in Picnik to edit your photo on the spot.
Click Set as profile photo, you have done it.