One of the primary values of Metasploit is that it is constantly being updated to provide exploits for the newest and most interesting vulnerabilities. As time goes on and patches are applied, a given exploit becomes less and less likely to work, so using the latest exploits is usually a very good idea. By routinely updating Metasploit (e.g., before every use), you give yourself the best chance of exploiting your targets successfully.
Older versions of Metasploit used a custom utility called msfupdate to grab the latest code, but as of Metasploit 3.0, msfupdate has been replaced by Subversion (http://subversion.tigris.org). Once you've downloaded Metasploit, you now keep it up to date simply by using your Subversion client of choice to "update" the Metasploit directory. For example, I update my Metasploit using the Unix command-line Subversion client called svn, which looks something like this:
[bryan@nereid metasploit-3.0] svn update
At revision 4532.This isn't a particularly exciting example because my Metasploit was already up to date, but then again, that's a good thing. If your Metasploit was in need of updating, you would see a list of file modifications and deletions more like this:
[bryan@nereid framework-3.0-beta-3] svn update
UU modules/nops/ppc/simple.rb
UU modules/nops/x86/opty2.rb
UU modules/nops/x86/single_byte.rb
UU modules/nops/nop_test.rb.ut.rb
A modules/nops/php
A modules/nops/php/generic.rb
UU modules/nops/sparc/random.rb
...
Updated to revision 4532.