Edit /etc/hbsd-update.conf
Replace all instances of 13 to 15. Replace all instances of master to
main. Use that newly modified config file with hbsd-update's -c
option.
Of course, you'll want to make sure you do your due diligence to make
sure things don't break such that data loss occurs. Updating two major
version numbers in one go might prove difficult for you.
Good luck!