Roles and permissions are what allow a user to have access to various features on the platform. Take a look at this article to view what each role/permission gives you access to.
To change a user's access level you need to navigate to their profile, either by using the search bar or using the people's module found on the left hand side in the navigation bar.
After navigating to their profile, click the settings icon located beneath their name, title and picture. This will take you to the user settings where you can change their access level.
You are able to alter the access level underneath the section that says 'Role and Permissions'. The role is on the left hand side whereas the permissions are on the right hand side. Changing the role changes the permissions that show.
Once you've selected the role and permissions that you wish for the user to have, that information will automatically update. You're good to go!