cat /usr/src/Makefile - смотрим там

# For individuals wanting to upgrade their sources (even if only a
# delta of a few days):
#
#  1.  `cd /usr/src'       (or to the directory containing your source tree).
#  2.  `make buildworld'
#  3.  `make buildkernel KERNCONF=YOUR_KERNEL_HERE'     (default is GENERIC).
#  4.  `make installkernel KERNCONF=YOUR_KERNEL_HERE'   (default is GENERIC).
#       [steps 3. & 4. can be combined by using the "kernel" target]
#  5.  `reboot'        (in single user mode: boot -s from the loader prompt).
#  6.  `mergemaster -p'
#  7.  `make installworld'
#  8.  `make delete-old'
#  9.  `mergemaster'                         (you may wish to use -U or -ai).
# 10.  `reboot'
# 11.  `make delete-old-libs' (in case no 3rd party program uses them anymore)
#
# See src/UPDATING `COMMON ITEMS' for more complete information.
#

проверяем, все ли операции сделаны
в частности mergemaster, который обновляет motd