I want to upgrade mongodb version(3.2 to 3.6) on prd environment W/O downtime. We have one primary node, one secondary node and one arbitrary. We are using ubuntu 16.04. So please guide me how can I achieve success on this. (3.2 -> 3.4 -> 3.6)
The steps for upgrading are well documented. You have to fully upgrade from MongoDB 3.2 to 3.4 before upgrading from 3.4 to 3.6. Replica sets allow rolling upgrades between adjacent major release series to maximise availability during the upgrade process.
To ensure success I would review the compatibility notes and upgrade instructions first, check your driver compatibility (and upgrade if needed), test in a representative staging/QA environment first, and take backups before and after significant admin changes.
Relevant documentation to review: