Edit in JSFiddle

   $('#id_button').click(function(){
       var sOld = $('#id_output').html();
       var sNew = sOld.replace(/Foo/g,"Bar");
       $('#id_output').html(sNew); 
        
    });

<div id="id_output">Das ist Foo im Foo-Text</div>
<button id="id_button">Test</button>