// Sets the HTML contents of the activeEditor editor
tinymce.activeEditor.setContent('<span>some</span> html');

// Sets the raw contents of the activeEditor editor
tinymce.activeEditor.setContent('<span>some</span> html', {format: 'raw'});

// Sets the content of a specific editor (my_editor in this example)
tinymce.get('my_editor').setContent(data);

// Sets the bbcode contents of the activeEditor editor if the bbcode plugin was added
tinymce.activeEditor.setContent('[b]some[/b] html', {format: 'bbcode'});