Upgrade method also available using :
1. csup or cvsup
use RELENG_6_4  tag
2. Using freebsd-update
# freebsd-update upgrade -r 6.4-RC1
# freebsd-update install
3. Download Iso file
Click here to download FreeBSD 6.4-RC1
For full news click here

Click here to read more from freebsd update to branch