I'm guessing you mean Unity 3.4.1? I am not testing with this version any more, though there weren't any relevant changes between 3.4.1 and 3.4.2, so my guess is you won't have to update to 3.4.2, but you could if you wanted. Once you download the unitypackage, simply go to Assets > Import Package > Custom Package... from within Unity and point it to the unitypackage to install it. Before you upgrade 2d toolkit or Unity, you could make a backup of your project folder just in case something goes wrong. It shouldn't, and I take a great deal of care in making sure things are backwards compatible, but its always better to be safe than sorry.