Replacing an array item (rather than appending)

config.yml

kind: SecretProviderClass
metadata:
  namespace: default
  name: db_credentials
spec:
  provider: aws
  parameters:
    objects: |
      - objectName: TO_BE_REPLACED_BY_YTT
        objectType: "secretsmanager"
        jmesPath: 
          - path: username
            objectAlias: dbusername
          - path: password
            objectAlias: dbpassword

overlay.yml

#@ load("@ytt:overlay", "overlay")
#@ load("@ytt:yaml", "yaml")
#@ load("@ytt:data", "data")

#@ def updates():
#@overlay/match by=lambda idx,left,right: "objectName" in left
- objectName: #@ data.values.db_secret_name
#@ end

#@overlay/match by=overlay.subset({"kind": "SecretProviderClass", "metadata": {"name": "db_credentials"}})
---
spec:
  provider: aws
  parameters:
    #@overlay/replace via=lambda a,_: yaml.encode(overlay.apply(yaml.decode(a), updates()))
    objects:

values-staging.yml

#@data/values
---
db_secret_name: db-secret-staging