You are allowed to modify or change the templates to adapt them to your needs. You are not allowed to remove the copyright information from the source code. You can remove the copyright information from the footer of the template.
However you are not allowed to resale or redistribute any version of the changed templates. You are not allowed to convert/port our templates to other CMS or any other platforms.
The demo content on all demo websites is for demonstrational purposes only and it's prohibited to use it.