In addition to the answers above, you can also tell pg_dump and other commands, which version to use via the cluster option. For instance,
$pg_dump --cluster 9.1/main
will target the 9.1 version.
Note that this makes use of pg_wrapper, which comes with postgresql-common and works on Ubuntu (Debian). Also note that the cluster (main in the example) might be different in your setting.
More information on pg_wrapper can be found in this DBA answerDBA answer.