To assign or change roles, go to Admin menu > People.

Locate the user you wish to assign a role to and check the box next to the user's name.

user selected from user list

In the Update Options select box, drop down the menu and select the role you wish to give this person, then click the Update button.

role selected from drop down menu

You will then receive a confirmation regarding the update to this user.  You will also see the role appear in the corresponding user's row in the table.

confirmation message appears at top of screen