To customise the themes 'Header' settings, from the store dashboard go to:
> Store Settings > Theme Options > Header

From within this page there are 3 features you can customise, these are:-
'Top flash: text' - the text featured at the top of your store / 'Top flash: link URL' - when clicked, sends you to your desired URL:

'Search box label' - the text featured in the search box before typing begins:

(*Don't forget to click 'Save Changes' after making alterations*)