thank you for using Bitnami. We configure Magento to use the single-user file permissions configuration, that means that only the daemon user and group can write in those files. You use the bitnami user when connecting to the instance using a SSH or a SFTP connection so you can't write in the Magento's folder. I suggest you follow these steps to copy new files to Magento
1a - If you are copying from your local computer to the cloud instance, copy the files to the home folder of bitnami (/home/bitnami)
1b - If you are downloading the files in the remote instance, download them in the home folder of the user
2 - Then use
sudo when copying the files
sudo mv /home/bitnami/the_file_or_folder /opt/bitnami/apps/magento/htdocs/and_the_folder_you_want_to_copy_the_file_to
sudo chown -R daemon:daemon /opt/bitnami/apps/magento/htdocs/
This way, you will copy the files and set the proper permissions after that.