The Bitnami Joomla solution already has PHP 7.x but old versions of the solution might not include this version of PHP. In your case, you will need to launch or install the latest version of the Bitnami solution and migrate the data from the old installation to the new one
Once you verify that everything is working properly, you could remove the old installation.
PHP 7.0 will be unsupported soon so we are moving our solutions to PHP 7.1. If the users want to upgrade PHP, they will need to follow the same procedure to migrate the application as upgrading the PHP component in the Bitnami solution is a complex task.
Sorry for the inconvenience