You can sometimes do "apt-get install postgresql-9.3=9.3.4-1"
But often the OS cannot work out where to get hold of the requested version.
Alternatively you can check in /var/cache/apt/archives.
If the .deb file for the downlevel version is still there (copy it out so it's not cleaned up in future), and you can do "dpkg -i postgres-9.3_9.3.4-1.deb"
Failing that, you can download the downlevel deb file from somewhere.