The account under which the appliction runs needs modify rights to the config file. If it runs under application pool identy you can set modify rights for the IIS_IUSRS group.
But I would recommend editing the config file in an text editor instead.
In Admin mode, system settings / generall, when I try to save it says
Access to the path 'C:\EPiServer\Sites\Nykoping.InternWeb\Nykoping.InternWeb.Community\web.config' is denied.
Any ideas why?