NetBSD project released 6.1.3, 6.0.4, 5.2.2 and 5.1.4 security/bugfix versions. Release notes can be found here, here, here and here. You can download them from usual places: check out mirrors or torrents. 6.1.3 release torrents for amd64 and i386 can be found in linuxtracker.com too. Update is encouraged for those who uses prior versions. Just grab the newest version if you are the new user.

I also found that NetBSD 5.0 branch no longer receives security updates, so you need to upgrade to newer ones.