In order to turn a learner to instructor or vice-versa you simply need to edit the user (1) and change its "user-type" field to Trainer-type (2). Note that you can modify or create additional user-types from Administrator->user-types that offer more or less privileges to users that have that user-type.
Furthermore, and since a user-type may have characteristics from multiple roles, you may want to set its role on a course. By default, a user is assigned to a course based as a learner. To modify that, edit the course, go to the Users tab and select the role of the user on that specific course (3).
If you do not see a dropdown there to change the role of a user on the course, it means that its user-type does not have characteristics from multiple user roles. You can either change his user-type or extend his assigned user-type with additional privileges.