Change link button text via javascript

 var mybutton = document.getElementById("<%=lbtnContinue.ClientID %>");
      mybutton.innerHTML = "";

Get/Set value of c# member variable via javascript

     var iTesting = "<%=m_iLensId%>";
     alert(iTesting);
     <%=m_iLensId=9%>
     var iTesting1 = "<%=m_iLensId%>";
     alert(iTesting1);