You can use the PGDG repo RPMs available here (just wget
and rpm -i). Then, you should probably use pg_dump[all] to
snapshot your databases/clusters, verify the dumps, uninstall the
old version of Postgres, and install the new version. Restore the
dumps, and you're good to go.
For a slightly faster alternative (which I haven't used), you could
use the upgrade utilities that I hear are available via the
postgresql-contrib packages to perform the above process in an
A third alternative, if downtime is absolutely unacceptable during
an upgrade dump/restore, is to start a second instance of Postgres
(the new version), and replicate your data from the old to the new
using Slony or similar. That will probably not be easy to set
up, however. I'd suggest just scheduling some maintenance, and using
option 1 or 2.