1. Select the Settings gearwheel icon. In the Settings drop-down menu, select 'Users'.
2. On the Users screen, you can view everyone in your organization. You can see and change each user's User Type.
If you have enough licenses and you wish to switch a user from a Full Access to a Contributor, find the appropriate user and click the drop down menu next to their email address, under 'User Type'. Select their new user type. Your changes will save automatically.
NOTE: Only Full Access users with Administrator power can change Plan Permissions.