You just need to upload the image you want into your server. For that you read this wiki page:
You need to upload your image on
/opt/bitnami/apps/discourse/htdocs/public/images. Next step is on the web interface, log in, go to the admin panel, Settings, and on the
logo url section write
images/mynewlogo.png, changing mynewlogo.png for your logo name.
I hope you can make it, let me know if you have any more questions.