there's a complete step by step guide installed with your installation. its called the freebsd handbook. But since its so easy, i'll enlighten you.
First cvsup your sources. For some stupid reason, the stable-supfile in freebsd5 by default, will try to cvsup the 4.X source, efectively deleting the 5.X source. idoits. anyways, to fix this, copy the /usr/share/examples/cvsup/stable-supfile to /root/ (or any other directory), and open it with your fav editor. Go the the line that reads
CODE
*default release=cvs tag=RELENG_4
and change it to read
CODE
*default release=cvs tag=RELENG_5_1
also change the default host.
save and exit and run cvsup stable-supfile. It will take a while to complete.
after this, do the following:
CODE
cd /usr/src
make cleandir
make buildworld
make buildkernel
make installkernel
make installworld
reboot
and you will have the latest stable release available.