Change the name of the site at the top of the header/tool bar

Hi there
I have a site https://www.ministylista.com It used to be called My Style Australia but recently changed the domain name and name of the blog! However I can not change the very top of the tool bar/header which is the name of the site. It currently reads My Style Australia at the very top and I want it to read mini stylista

I have changed my username and name of the blog! IS it to do with the inital set up? Is it something my host can help me with or do I need to start over and reload wordpress again? HELP