This may help you out:
http://www.postgresql.org/docs/9.1/static/upgrading.html
or else you can follow these steps:
As root:
su - postgres
pg_dumpall > dump.sql
exit
cp ~postgres/dump.sql /root/
Now you can safely remove the postgresql-8.4 and install postgresql-9.1:
aptitude purge postgresql-8.4
aptitude install postgresql-9.1
Next check the postgresql configuration in /etc/postgresql/9.1/main. If you make any changes, make sure to restart postgres with /etc/init.d/postgresql restart.
Postgresql 9.1 is now up and running, let's import our data back into it.
su - postgres
psql < dump.sql
That's all. You're now fully upgraded to PostgreSQL 9.1
edit:
similar question was asked here:
https://askubuntu.com/questions/66194/how-do-i-migrate-my-postgres-data-from-8-4-to-9-1