To upgrade to OpenBSD OSREV from a previous version, you should follow the
instructions on installing the system beginning with using the MacBSD
Install Utility.  All you need do is copy each of the distribution sets in
over your current system from the MacOS.

However, you should not copy the etc{:--:}OSrev.tgz file into the system.
Instead, download it while running OpenBSD and put it in /tmp or some other
harmless place and unpack it with a command like 'tar zxvf etc{:--:}OSrev.tgz'.
You will then have an etc directory with current system configuration files
in it.  Use this to carefully upgrade your files in the running /etc and
reboot the system.  Also, it is highly recommended that when upgrading your
kernel binary (/bsd) you backup the old, working kernel and extract
bsd-generic.tgz into / only from OpenBSD.