pg_upgrade is a utility for upgrading from a previous Postgres release without reloading all the data. Not all Postgres release transitions can be handled this way. Check the release notes for details on your installation.
Upgrading Postgres with pg_upgrade
Back up your existing data directory, preferably by making a complete dump with pg_dumpall.
Then do:
$ pg_dumpall -s >db.out
to dump out your old database's table definitions without any data.
Stop the old postmaster and all backends.
Rename (using mv) your old pgsql data/ directory to data.old/.
Do:
$ make install
to install the new binaries.
Run initdb to create a new template1 database containing the system tables for the new release.
Start the new postmaster. (Note: it is critical that no users connect to the database until the upgrade is complete. You may wish to start the postmaster without -i and/or alter pg_hba.conf temporarily.)
Change your working directory to the pgsql main directory, and type:
$ pg_upgrade -f db.out data.old
The program will do some checking to make sure everything is properly
configured, and will run your db.out script to recreate all the databases
and tables you had, but with no data. It will then physically move the
data files containing non-system tables and indexes from
data.old/ into the proper
data/ subdirectories, replacing the empty data files
created during the db.out script.
Restore your old pg_hba.conf if needed to allow user logins.
Stop and restart the postmaster.
Carefully examine the contents of the upgraded database. If you detect problems, you'll need to recover by restoring from your full pg_dump backup. You can delete the data.old/ directory when you are satisfied.
The upgraded database will be in an un-vacuumed state. You will probably want to run a VACUUM ANALYZE before beginning production work.