To change the owner of a content item start by clicking the + icon next to the content item owner's profile pic, bringing up the Add and Remove Team Members overlay.
Hover over the user's name that you wish to make the owner. If they have the right user permissions to be a content owner then a blue make owner option will show to the right of their name, click this option.
This will change the owner within the Add and Remove Team Members overlay. To confirm the change click the blue button at the bottom that will show the number of team members being assigned plus verbiage informing you a content item owner change will take place. That's it! Owner changed!