1. Stop Evergreen and disconnect clients from database.  When you stop Evergreen, the clients will be disconnected automatically, in an abrupt way.  It might be better to disconnect the clients first, and then after the users are properly cleared off and their changes saved, stop Evergreen.
As ''postgres'' user

''pkill -f 'postgres: postgres <database_name> ''

''pkill -f 'postgres: postgres evergreen''
 
2. Make your life easier by adding python-software-properties

''sudo apt-get install python-software-properties''
 
3. Add the backport repository to your repository sources list:

''sudo add-apt-repository ppa:pitti/postgresql''

4. Update the sources list on your server.
                                
''apt-get update''

5. Check that you are getting all your databases

''psql ''

''postgres # \l+ ''

''#that is a lower-case L, and the plus sign gives you extended information about the databases you are running.''

6. As the ''postgres'' user, backup your PostgreSQL databases.
 
''pg_dumpall > data.sql  # This is the simplest way to get everything connected with the current instance of PostgreSQL''

''# or individually..''

''pg_dump -U username -W database_name > db.sql ''

''# for instance..''

''pg_dump -U evergreen -W evergreen > evergreen.sql''

''pg_dump -U rsyslog -W Syslog > Syslog.sql''

''#and so on''

7. As root, remove PostgreSQL 8.4 and install 9.0.
                 
''aptitude remove postgresql-8.4''

You could use ''aptitude purge postgresql-8.4'' but because this removes the configuration files of postgresql-8.4 as well as the application packages, it may make your life more difficult.  You may wish you had your configuration files for review later in the upgrade process.

''aptitude install postgresql-9.0 postgresql-client-9.0 postgresql-contrib-9.0 postgresql-plperl-9.0''

8. as the postgres user, recreate empty evergreen databases with required language supports.
If you do not have Evergreen-ils on the system yet, you can skip this step and go on to step 9.

''dropdb evergreen''

''createdb -T template0 --lc-ctype=C --lc-collate=C -E UNICODE evergreen''

''createlang plperl   evergreen''

''createlang plperlu  evergreen''

''createlang plpgsql  evergreen''

''psql -f /usr/share/postgresql/9.0/contrib/tablefunc.sql evergreen''

''psql -f /usr/share/postgresql/9.0/contrib/tsearch2.sql  evergreen''

''psql -f /usr/share/postgresql/9.0/contrib/pgxml.sql     evergreen''

9. Update the 8.4 cluster (with evergreen database) to 9.0.  
For this to work properly, and not get the error “Target already exists 9.0 main,” you will need to move(rename) or delete the following directory:

''/var/lib/postgresql/9.0/main''

and

''/etc/postgresql/9.0/main''

The rationale for moving(renaming) these directories, rather than straight-out deletion, is you may well discover that your ''pg_hba.conf'' file is empty of all your changes when you complete the ''pg_updatecluster'' command.
As the ''root'' user: 

''sudo mv /var/lib/postgresql/9.0/main /var/lib/postgresql/9.0/oldmain''

''sudo mv /etc/postgresql/9.0/main /etc/postgresql/oldmain''

You may need to do the following command as ''root'', as it is required to write into ''/etc/postgresql'' which is owned by ''root''
                                
''pg_upgradecluster 8.4 main''

10. as the ''postgres'' user, load data from backup into PostgreSQL 9.0.
                                
''psql < data.sql  # This command loads your databases into the new version of PostgreSQL''

''# If you are updating specific databases, rather than upgrading the entire collection...''

''psql -U username -W database < db.sql''

11. Restart Evergreen and Apache.

12. Test by logging into psql, checking the postgreSQL version and running a simple SELECT query.
        
''psql -U evergreen ''
                      
''evergreen=# select version();''

''evergreen=# select count(*) from biblio.record_entry;''

13. Check ''pg_hba.conf'' and to make sure your configurations are right and the database is allowing connections from the machines, IP addresses or IP ranges you expect, and  ''postgresql.conf'' to make sure your PostgreSQL server is listening on the right ports.  If everything is communicating properly, it is safe to delete the old folders.

''rm -rf /var/lib/postgresql/9.0/oldmain''

''rm -rf /etc/postgresql/9.0/oldmain''

''rm -rf /var/lib/postgresql/8.4''

''rm -rf /etc/postgresql/8.4''

**Note:** There is no need to be in a huge hurry to delete these old files.  It might be just as wise to hold on to them until you have used Evergreen for a month or two.  It is easy to remove them later, but very difficult to recreate them if you delete them too early.