I did it without these recommendations:
$
sudo pkg-static del -f pkg
Password:
pkg-static: Warning: Major OS version upgrade detected. Running "pkg-static install -f pkg" recommended
Checking integrity... done (0 conflicting)
Deinstallation has been requested for the following 1 packages (of 0 packages in the universe):
Installed packages to be REMOVED:
pkg-1.10.5_5
Number of packages to be removed: 1
The operation will free 12 MiB.
Proceed with deinstalling packages? [y/N]: y
[1/1] Deinstalling pkg-1.10.5_5...
[1/1] Deleting files for pkg-1.10.5_5: 100%
nat:[~]$
sudo pkg upgrade
The package management tool is not yet installed on your system.
Do you want to fetch and install it now? [y/N]: y
Bootstrapping pkg from pkg+
http://pkg.FreeBSD.org/FreeBSD:12:amd64/quarterly, please wait...
Verifying signature with trusted certificate pkg.freebsd.org.2013102301... done
Installing pkg-1.10.5_5...
Extracting pkg-1.10.5_5: 100%
Updating FreeBSD repository catalogue...
pkg: Repository FreeBSD has a wrong packagesite, need to re-create database
Fetching meta.txz: 100% 944 B 0.9kB/s 00:01
Fetching packagesite.txz: 100% 6 MiB 2.2MB/s 00:03
Processing entries: 100%
FreeBSD repository update completed. 31468 packages processed.
All repositories are up to date.
Updating database digests format: 100%
Checking for upgrades (119 candidates): 100%
Processing candidates (119 candidates): 100%
The following 118 package(s) will be affected (of 0 checked):
Installed packages to be UPGRADED:
ca_root_nss: 3.40 -> 3.41
Installed packages to be REINSTALLED:
unixODBC-2.3.7 (ABI changed: 'freebsd:11:x86:64' -> 'freebsd:12:x86:64')
unbound-1.8.0 (ABI changed: 'freebsd:11:x86:64' -> 'freebsd:12:x86:64')
tshark-2.6.3 (ABI changed: 'freebsd:11:
etc etc