Bitnami Stacks are installed in AWS instances using the root user, so a normal user (like bitnami, ubuntu or user) would not be able to perform changes to configuration files.
In order to modify those files you should enter the system as root user.
Using ssh you can connect to
If you are already logged in you can execute
sudo su, type your password and then you are logged as root user and you will be able to modify those files.
I hope this will help you. Tell us if you have any further questions.