how to upgrade postgresql 9.0.4 to latest with minimum downtime I have master-slave-slave setup for postgresql 9.0.4 in our production setup. 
We need to upgrade it to latest (11.X), so my question is what would be the procedure to do so with as little downtime as possible.

Also what options (method wise) i have to upgrade it.
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.