Click on your profile picture (top right hand corner of the navigation).

Once on your profile page click the edit action button under your profile picture.

This will take you to the profile image editor. 

Click the upload button to open your file upload window, select a file to open. Note: File size is limited to 8Mb.

Once uploaded to the profile image editor you can change the scale (with the bottom slider) and the circular crop by dragging the circular frame around your image. 

Click the save/tick action button to save changes.