Adding css class

How would I go about changing the css class of the widget? I’m guessing it would be within the php file itself, but I’ve tried two places and it keeps messing everything up.

What I’m trying to do is get the initial div class to look like: <class=”postswidget widget widget_text”>

rather than
<class=”widget simple_tweet_widget”>

I need to remove the simple_tweet_widget class and add postswidget and widget_text

http://wordpress.org/extend/plugins/twitget/