how to upgrade postgresql 9.0.4 to latest with minimum downtime
The fastest way will be pg_upgrade with the -k option.

This way. you can get away with a few minutes down time, which is required to move the database metadata.

If you cannot afford a few minutes down time, you will have to resort to a trigger based replication method like Slony. This way, you can replicate the database to a current version and switch the application over once replication has caught up.