To upgrade to OpenBSD 2.1 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 etc21.tar.gz 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 etc21.tar.gz'.
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 hihgly recommended that when upgrading your
kernel binary (/bsd) you backup the old, working kernel and extract
bsd21.tar.gz into / only from OpenBSD.