Please find the appropriate upgrade guide on selecting the application server and database server. Please find the following link for the combination of the Postgres DB with the tomcat application server.
@GARCM Thank you for sharing the details. In our case we would be updating from 8.1.4 to 8.5. So I was looking for Pega update approach documents for both in place and out of place update. Also Pega Platform changes with any update impact from 8.1.4 to 8.5