get source code

$git clone -b releng/13.0 --depth 1 https://git.freebsd.org/src.git freebsdsrc

rebuild world

cleanup

#cd /usr/obj
#chflags -R noschg *
#rm -rf *

build the world (compiler and the others) (with 32 cores)

#cd /usr/src
#time -l make -j 32 buildworld
--------------------------------------------------------------
>>> World build completed on Wed Jun  2 21:56:15 PDT 2021
>>> World built in 684 seconds, ncpu: 32, make -j32
--------------------------------------------------------------
      683.96 real     17918.15 user       752.25 sys

build a new kernel (with 32 cores)

#time -l make -j 32 buildkernel
--------------------------------------------------------------
>>> Kernel build for GENERIC completed on Wed Jun  2 21:59:10 PDT 2021
--------------------------------------------------------------
>>> Kernel(s)  GENERIC built in 65 seconds, ncpu: 32, make -j32
--------------------------------------------------------------
       65.28 real      1647.80 user       127.65 sys

install the new kernel

#make installkernel

enter the single-mode

#shutdown now

ONLY for ufs

#mount -u /
#mount -a -t ufs
#swapon -a

ONLY for zfs

#zfs set readonly=off zroot
#zfs mount -a

initial configuration file update for next installworld

#mergemaster -p

install the new world

#cd /usr/src
#make installworld

update other configuration files

#mergemaster -iF

delete any obsolete files

#make delete-old

reboot

#reboot

delete any obsolete libraries to avoid conflicts with newer ones.

#make delete-old-libs