In order to change the URL of your Discourse installation, you can follow these instructions: https://wiki.bitnami.com/Applications/BitNami_Discourse#How_to_change_the_default_hostname_of_the_application.3f
Note that if your Discourse site was launched from the cloud, you might find it resets your hostname each time you restart your instance. To avoid this, rename the bnconfig file (mentioned in the link above) to something like this:
sudo mv /opt/bitnami/apps/discourse/bnconfig /opt/bitnami/apps/discourse/bnconfig.old
I hope it works!