$("#thebutton span").text("My NEW Text");
<input type='button' value='Add' id='btnAddProfile'>
$("#btnAddProfile").attr('value', 'Save'); //versions older than 1.6
<input type='button' value='Add' id='btnAddProfile'>
$("#btnAddProfile").prop('value', 'Save'); //versions newer than 1.6
<!-- Different button types-->
<button id='btnAddProfile' type='button'>Add</button>
$("#btnAddProfile").html('Save');