[vermaden]

First read FreeBSD Handbook section here:
http://freebsd.org/handbook/makeworld.html

Then its like that:

# cd /usr/src
# make buildworld
# make kernel KERNCONF=GENERIC KODIR=/boot/testing
# nextboot -k testing
# sync
# reboot

--- (if new kernel works)
# cd /boot
# mv kernel OLD
# mv testing kernel
# cd /usr/src
# mergemaster -p
# make installworld
# mergemaster -iU
# sync
# reboot

--- (optional)
# make delete-old
# make delete-old-libs