For upgrading Postgres on AWS EC2 machines, the steps would be the same as that of on-premises Postgres deployment.

1. Check the release note of the latest version for any incompatibilities.
2. Take the backup of the Postgres database that you plan to upgrade.
3. Test upgrade in UAT environment and test the application against the latest upgraded version of Postgres for function and performance evaluation
4. Perform the upgrade in the production environment and identify the cut-off time for the application.