Administrators can update the permissions of a user or users by heading to Settings which can be found at the bottom of the sidebar.
Once you're on Settings, find the user you want to edit and click the pencil icon.
This will take you to Set permissions which allows you to change the role or update the permissions of the user. When you are happy with your selection, click Apply changes.
You can also update the permissions of multiple users at once by using the multi-select on the far left of the table and then clicking Edit permissions.