Change <td> attributes via DOM.. How to??

I'm trying to created a click tab effect but need help. As it is I have two table cells with a class and onclick as so:

<td class="selectedtab" onclick="switchtab(0)" tab="tab1"><td class="tab" onclick="switchtab(1)" id="tab2">

What I'm trying to do is create a switchtab function that will change to class and onclick to opposites, like this:

<td class="tab" onclick="switchtab(1)" tab="tab1"><td class="selectedtab" onclick="switchtab(0)" id="tab2">

All I have is this:
function switchtab(select) {
     getElementByID(tab1)
              ...
     getElementByID(tab2)
              ...
}

How do I switch class and onclick attirbutes like this. Pls help. Thanks in advance for all the help.