https://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/versions.html#versions-13

sysctl kern.osreldate
/usr/src/sys/sys/param.h
#define __FreeBSD_version 1300102

Modify the /usr/src/sys/sys/param.h  __FreeBSD_version to the value get by sysctl kern.osreldate in the OS.