To upgrade to OpenBSD 2.3 from a previous version start with the general
instructions in the section "Installing OpenBSD".

If you are upgrading with the miniroot or the new single floppy image,
select the (U)pdate option rather than the (I)nstall option at the prompt
in the install process. 

If you are using the old multi-floppy installation procedure, use the
"upgr22.fs" floppy rather than the "inst22.fs" floppy when prompted to
put in the filesystem diskette.  When you arrive at the shell prompt,
run the "upgrade" script rather than "install" script.

The upgrade script will use the existing disk partitions to install the
new system in, and also preserves files in `/etc' which you are likely
to have customized since a previous installation.