Version 3.4.2 fixes a few security issues and contains some security hardening.
The vulnerabilities included potential privilege escalation and a bug that affects multisite installs with untrusted users.
These issues were discovered and fixed by the WordPress security team.
- Fix some issues with older browsers in the administration area.
- Fix an issue where a theme may not preview correctly, or its screenshot may not be displayed.
- Improve plugin compatibility with the visual editor.
- Address pagination problems with some category permalink structures.
- Avoid errors with both oEmbed providers and trackbacks.
- Prevent improperly sized header images from being uploaded.
Source: http://wordpress.org/news/
If you don’t understand why its important to update your installation, read this post about what can happen if you don’t.
Update WordPress Manually
Normally you can easily update WordPress automatically from your admin panel.
Go to Dashboard > Updates and follow the simple instructions.
Here’s a tutorial on how to update WordPress manually if you can’t get the job done automatically.