Can you please change the Postgres packages to use postgres94,
postgres95, postgres96, etc. names? Each version is incompatible with
the previous. This will allow installing Postgres in separate binary
directories to enable upgrades. Alternatively, please increase the
'previous version' history to allow installing old versions. Setup only
has 1 previous version, and if the current version is 9.5.2 and the
previous version is 9.5.1, then there is no way (or very difficult) to
install 9.4!

http://serverfault.com/questions/532379/how-do-i-upgrade-postgres-database-in-cygwin