Follow the direction for upgrading mysql from the DA knowledgebase. Have you looked there? It tells you how to download the distribution of mysql that you want to use from the MySQL website then it tells you how you should install it for Freebsd.
mysql site support FreeBSD 6.x Only not support freeBSD5.x
FreeBSD (TAR format) downloads (platform notes)
Note: FreeBSD 6.0 x86_64 users are advised to upgrade to FreeBSD 6.1to avoid problems running the MySQL server (FreeBSD bug 95127)
FreeBSD 6.x (x86) Standard 4.1.21 29.3M Pick a mirror
MD5: 92eb77471ad280ecfb3768303ab1fd48 | Signature
Max 4.1.21 42.8M Pick a mirror
MD5: 4b51c1d909ef294d20484324e187ad4b | Signature
Debug 4.1.21 30.4M Pick a mirror
MD5: c885b570081e3d262b9f8667f57e49a3 | Signature
FreeBSD 6.x (x86_64) Standard 4.1.21 31.0M Pick a mirror