Using jQuery
You can set the class (regardless of what it was) by using .attr(), like this:
$("#td_id").attr('class', 'newClass');
If you want to add a class, use .addclass() instead, like this:
$("#td_id").addClass('newClass');
Or a short way to swap classes using .toggleClass():
$("#td_id").toggleClass('change_me newClass');
Here’s the full list of jQuery methods specifically for the class attribute.