On Windows, simply stopping the postgresql service, then running postgresql-9.3.1-1-windows-x64.exe on top of the existing 9.3.0 works. No uninstall necessary. Of course, a backup is recommended.
Clear, explcit documentation for the update procedure on Windows is absent. Note that the documentation link provided by @dezso has been moved in the 9.3 manual to
Chapter 17 Server Setup and Operation, section 17.6 Upgrading a PostgreSQL Cluster
in the 9.0 manual the location was located in
Chapter 24 Backup and Restore, section 24.4 Migration Between Releases
The best source of Windows installer information was the Enterprise DB forums. Here are some posting I found that addressed the question...
2010 April - What's the right way to upgrade to a new version of PostgreSQL
2010 January - Upgrading to 8.4.2 from 8.4.1
2009 September - Upgrade procedure when using one-click installer?