How to upgrade FreeBSD 11 to 12

First note down the FreeBSD version by running the following commands:
freebsd-version
uname -mrs

Make sure both FreeBSD base and packages are updates:
freebsd-update fetch install
pkg update && pkg upgrade

Upgrading from previous releases of FreeBSD 11.2 to 12.0 using binary method

Type the following freebsd-update command:
freebsd-update -r 12.0-RELEASE upgrade

Once everything downloaded, merged in a directory, it is time to commit changes to the disk, run:
freebsd-update install
reboot
freebsd-update install

Now base system has been updated. It is time to update all binary packages too. Simply run the following pkg command:
pkg-static install -f pkg
pkg update
pkg upgrade