If you need to make an Editor a Reader, or a Learner an Admin, you can simply modify their role by following these instructions.
Note: You must be an Admin to change another user's role.
Click the user's name
Select a new role
Confirm permissions are accurate
Navigate to Site Permissions to review whether this user's permissions will allow them to author content in the site's they need.
You can modify the contributor's permissions individually: Creating Contributor Permission Rules for a User