dave, simply type in thunderbird into your favorite search engine and download sources will pop up all over, find a source you trust and download it.
If you are running windows just simply left click on your "start" button on lower left, type in THUNDERBIRD in the search box and if you have TBIRD preinstalled it will then show up for you on the search results, if not then download Tbird and install, looks like the NEW THURNDERBIRD 3 edition is out, I might upgrade to it.
later,
Cyprs