You should access via SSH to your server (documentation). Once you access, you could edit your file with your favorite editor (e.g. nano, vim or emacs). Prepend
sudo to your command to have superuser permissions. Here, I will add an example about how to do it from command line (using emacs in this case):
ssh -i MY_PRIVATE_KEY bitnami@MY_IP
sudo emacs /opt/bitnami/apps/mantis/htdocs/config_defaults_inc.php
It will open
/opt/bitnami/apps/mantis/htdocs/config_defaults_inc.php file and let you edit it.
I hope it helps.