First navigate to the course where you want to change the role of a course participant. Then:

1.  Navigate to Communication

2.  Click Classlist

In the Classlist page you will see a table with all the participants in your course:

3.  Click the Selectbox in front of  the users you want to change their role

4.  Click Enrolment

5.  Click the drop down menu in the New Role column. Select the new role, for example Teaching Assistant

6.  Click Save

The role of the selected users has been changed!