Download our latest Official version of the Second Life viewer, version 1.20.16:
http://secondlife.com/support/downloads.php
(Alternately, you should also see a prompt on your startup screen that says "A new version is available..." with the ability to download it directly from your startup screen.)
If you are a participant in testing the 1.21 Release Candidate:
Residents who were using RC2 will be automatically prompted (required) to upgrade to RC3 when you try to log in. This will let you benefit from the latest bugfixes under consideration for the next version of Second Life, 1.21.
http://secondlife.com/support/downloads.php#download-Testviewers