How to upgrade Dokku's PostgreSQL?

Unfortunately, there is no easy way to accomplish this except for forking the plugin. With Dokku, everything is running inside of Docker containers including the database. If you look at the install script for Kloadut/dokku-pg-plugin, you’ll see that it builds a Docker image based on this Dockerfile:

FROM	ubuntu:trusty
MAINTAINER	kload "kload@kload.fr"

# prevent apt from starting postgres right after the installation
RUN	echo "#!/bin/sh\nexit 101" > /usr/sbin/policy-rc.d; chmod +x /usr/sbin/policy-rc.d

RUN apt-get update
RUN	LC_ALL=C DEBIAN_FRONTEND=noninteractive apt-get install -y -q postgresql-9.3 postgresql-contrib-9.3
RUN rm -rf /var/lib/apt/lists/*
RUN apt-get clean

# allow autostart again
RUN	rm /usr/sbin/policy-rc.d

ADD	. /usr/bin
RUN	chmod +x /usr/bin/start_pgsql.sh
RUN echo 'host all all 0.0.0.0/0 md5' >> /etc/postgresql/9.3/main/pg_hba.conf
RUN sed -i -e"s/var\/lib/opt/g" /etc/postgresql/9.3/main/postgresql.conf

So the only way to run a different version of Postgres would be to fork the plugin and have it use a different Dockerfile.