How to replace/edit alertmanager.yml in PMM with our custom alertmanager?

PMM is not accepting the custom alert manager config in /etc/alertmanager.yml. It seems its uneditable. I want to edit it and add alert conditions and slack notifications to it.

/etc/alertmanager.yml
# Managed by pmm-managed. DO NOT EDIT.
---
global:
    resolve_timeout: 0s
    smtp_require_tls: false
route:
    receiver: empty
    continue: false
receivers:
    - name: empty
templates: []`