To upgrade to OpenBSD OSREV 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. 

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.