Only Teachers, Authors and Managers are able to promote Students into Teaching assistants (Students are not able to do this). To see what kind of access your Teaching assistant will get, please visit this article here, where we compare the functionalities of a Teacher and a Teaching assistant.
Firstly, make sure that your Teaching assistant is already enrolled in your class as a Student. You can invite them by sending them an access code. You can read more about it here.
Go to the tab Teacher- Users, and you will see the overview of Teachers and Students in your class. Find the user that you wish to upgrade, and click on "student" next to their name. Click on "teaching assistant" to give them this title.
Once you have done this, you will see the new teaching assistant under "Teachers".
Please note: a person can be a Student in one class, and a Teaching assistant in another class.
How to take away somebody's Teaching assistant access?
Remove the user altogether by clicking on the red trash can next to their name. Then add them to the class again, by searching for their name in the search bar on the right. They will be added as regular Students, without the Teaching assistant access.