You can edit airflow_local_settings.py in Astronomer Enterprise, but the change will apply your airflow_local_settings for all deployments of airflow that you set up with this instance of Astronomer. If you are careful to make sure that your change will work with all your deployments (be careful about different versions of Airflow), or just have a single Airflow deployment, or spin up a whole new Astronomer instance when you want to make deployments of other versions of Airflow then this will not be a problem.
Caveats aside, here you would make this change:
You can append your code in this section of the config.yaml that you use to launch your Astronomer instance. Read the existing value carefully and make sure that your code does not conflict with the code that Astronomer has as a default; this code is required to operate your instance correctly.