Witaj Zbigniew,

you installed Quickstart with template, not only template - becuase standalone template do not have demo content inside, just to clarify ;)

Methods are this same since Joomla 1.0.x (2005):

1. Check all demo articles and change author of them, inside Article Edit > Publishing (tab) > Created By
2. You can delete all demo articles, and create a new one - with real content and you as author etc.
3. You can also create a new Joomla user then using PHPMyAdmin you can change his ID from XX to 962.